Something else ''futuristic'' happened in the fall of 1968: Encouraged by the people at Columbia Records, a 29-year-old composer named Walter Carlos completed an extensive project that involved transcribing for electronic music synthesizers various pieces of J.S. Bach.

For Carlos this was a labor of love, a sincere effort to demonstrate beyond the shadow of a doubt that electronic instruments could be played as expressively as their traditional counterparts. For Columbia the project was a commercial gamble, a profit-minded shot in the dark at a fickle public with a famous appetite for novelty. Both goals, it seems, were reached. Within months of its issue the recording found its way into the hands of more than 400,000 customers; a few years later it made history as the first classical album ever to sell a million copies. ''Switched-On Bach'' (Columbia MS 7194) is as remarkable today as when its vinyl was fresh. And listeners who have it tucked away somewhere on their shelf now have good reason to dust it off and reacquaint themselves with its charms.

In honor of the upcoming 25th anniversary, Carlos has completely reworked the material using state-of-the-art equipment. The updated treatment is on the Telarc label (CD-80323); with an eye and ear to the next century, it's titled ''Switched-On Bach 2000.''

To find the new recording in the store, shoppers need to ask the salesperson for the latest release by Wendy, not Walter, Carlos. Carlos did not announce the fact until 1979, but in 1972 he underwent a gender change. With this, too, he assured himself a place in the history books; among well-known musicians, Carlos is the only acknowledged transsexual. Portraits of Wendy Carlos decorate most of the recordings the composer made after 1979. Before that the album covers are relatively bare, adorned only with drawings and pictures of models wearing 18th-century costumes. Indeed, observers of the electronic music scene have long been wondering what Carlos looked like before his surgery; except for a curiously bearded, moustached mug shot that accompanied a 1979 interview in Playboy magazine, photographs of Walter Carlos are practically nonexistent. However, the Post-Dispatch archives contain a half-dozen excellent photographs that document Carlos' appearance with the St. Louis Symphony in 1969.

The event was by any standards unusual, but appropriate to a brand new concert hall whose inauguration opened the door to a promising future. In retrospect, the event was rare, perhaps even unique. Carlos' ''Switched-On Bach'' recording had been the result of more than 1,000 hours of painstaking work in a custom-designed electronic music studio. According to Post-Dispatch reports, the Powell Hall concert on Dec. 6, 1969, marked the first time Carlos attempted a ''live'' performance of his synthesizer arrangements. If Carlos subsequently gave a ''Switched-On Bach'' concert anywhere else in the world, it is not mentioned in any of the articles and interviews that make up the extensive Carlos literature.

The Post-Dispatch photos show an androgynous-looking composer dressed in Edwardian clothes. More important, they illustrate the bulky contraption with which Carlos worked his magic. Once upon a time, such a thing was called simply a synthesizer. Today, surrounded as we are by devices dependent on digital information channelled through microprocessors, it is described as an ''analog'' synthesizer. Like modern electronic musical instruments, the old ones synthesized sounds by combining electrical signals of various wave-forms and frequencies and then using amplified versions of those signals to set into vibrant motion the diaphragms of a loudspeaker.

The essential differences between analog and digital synthesizers have to do both with the way the sounds are constructed and the way the sounds are modified once they've been triggered. The digital synthesist can create sounds whose harmonic structure is enormously complex and whose envelopes - the ''shape'' of the sound's beginning, middle and end - are enormously fluid. He cannot, however, change the quality of those sounds during ''real time'' performance. All of his sounds are fixed, written as computer files and committed to memory on a computer's storage disc. Files can be switched in an instant to produce very colorful transitions, but any single sound can be changed only by re-programming. The analog synthesist, on the other hand, can make alterations in a sound while that sound is being played. His vocabulary of wave-forms and envelopes is relatively limited, but not his potential for fiddling with knobs and sliders.

Manipulated subtly or drastically, the various controllers afford the synthesist hands-on access to almost all the ingredients that constitute an electronically generated sound. The controllers are analogous to a sound's characteristics; hence the term ''analog.'' A digital synthesizer is typically a polyphonic instrument, capable of producing more than a single note at once. Most analog synthesizers available today are also polyphonic, and so are most so-called hybrid synthesizers that conveniently feature analog controllers and digital programmability. In the old days synthesizers were monophonic; like a clarinet or a trumpet, they could play only one note at a time. What's more, they were not - as are most modern synthesizers - ''touch-sensitive''; no matter how hard a performer pressed on the key that activated the sound, the note would speak only at whatever volume level was at that particular moment specified by the controllers. On top of that, electrical signals emanating from the old analog synthesizers were as ephemeral as the instruments' sounds; a computer-driven device called a sequencer can ''listen to,'' and memorize, practically every nuance of a modern synthesizer performance, but in the 1960s a synthesizer's output could travel only to the loudspeakers.

It is easy enough to see how Wendy Carlos came up with the sonically rich, musically sophisticated interpretations - with ''historically correct'' tunings and a full panoply of Dolby ''Surround Sound'' effects - that make up her new ''Switched-On Bach 2000'' album. But how could Walter Carlos, 24 years ago, accomplish something that sounds equally valid?

As noted, the original ''Switched-On Bach'' was made in a studio, and the material was never intended for presentation in a concert hall. With only a monophonic instrument at his disposal, Carlos had no choice but to unravel Bach's elaborate counterpoint and then record - on a reel-to-reel tape machine - one thread of music at a time. He had collaborated with Robert Moog, the inventor of the first commercially available synthesizer, to develop a primitive version of the touch-sensitive keyboard, and thus he was able to lay down phrases that had considerable dynamic range. To coordinate one part with another, though, he had to rely on a metronomic ''click track'' that later would be erased from the mix. Melodic lines that changed color rapidly were recorded piecemeal, sometimes one measure at a time, and then spliced together. Brilliantly virtuosic passages, in which ''Switched-On Bach'' abounds, were usually recorded at half-speed.

Working with a pair of monophonic synthesizers, Carlos would have been able to perform in public any of the three two-part inventions featured on ''Switched-On Bach.'' The performances would have had to be duller than on the recording, though, for with his fingers playing the notes he could not have manipulated the synthesizers' tone controllers. Even with an assistant to do the knob-twiddling, it would have been impossible for Carlos actually to play the ''Brandenburg Concerto No. 3'' that is the climax of the ''Switched-On Bach'' album. For all that, the ''Brandenburg Concerto'' was on the St. Louis program in 1969, along with a few Scarlatti sonatas, several works by Monteverdi and the complete ''Water Music'' of Handel.

In most of these, the orchestra (led by then assistant conductor Leonard Slatkin) alternated movements with Carlos; this was necessary, the reviewer noted, ''because the synthesizer could not be properly attuned to the pitch of the orchestra.'' And in everything that Carlos did, a single line of monophonic synthesizer performance was accompanied by as many pre-recorded synthesizer tracks as was necessary. The limitations being as severe as they were, it is no wonder that Carlos did not pursue a career in the concert hall.

Carlos did, however, continue to blaze the trail that began with ''Switched-On Bach,'' and for that listeners everywhere should be grateful. Although never an especially original composer, Carlos has always been a leader in the field of synthesis and the ''orchestration'' of electronic sounds. Many of the significant developments in electronic music technology over the last quarter-century had their start on Carlos' workbench; in the ever-broadening electronic music mainstream, the basic gestures and palettes are for the most part those that Carlos invented.