DRESSING UP gets a dressing down this season.

The super snappy, fitted, uptight three-piece suit gives way to an easier, more comfortable, more classic way for men to dress. The suit is now at ease, the fit looser, the shoulders softer, the lapels narrower, the shirt collar trimmed and the necktie slimmed down. Tweeds take the place of smoth finished wools, corduroy (particularly the narrow wale) shows up for suits or sportswear. Knitted or wool ties are abundant. Hats and caps are worn casually. And even saddle shoes reappear, done in suede and softened in color.

A lot of it appears like something that's been in the back of the closet a long time. But don't expect to recycle your 1950s clothes for real. The men's wear industry would never permit that. This season's looser fit is not as baggy as before, the narrower lapels and silhouette have taken on a new shope and are a compromise between recent and old styles.

Clothes acquire versatility from the toning-down of the newer European cuts and the spiffying-up of the older ones. The suit easily takes a sweater or contrast vest for the office; for some the sweater can replace the jacket.

This season's essential extra is the extra jacket. It can team up with matching pants as a suit,or top lots of different trousers, jeans included, for both work and weekend wear. Undergirded with a sweater and vest, it can take the place of a topcoat.

The high cost of good men's clothes today requires such versatility, though the classic styling of the current crop of clothes makes them a worthy investment. It's the fabrics, the Harris tweeds, the Donegals, the camel hair, the SHetlands and other fine wools that have pushed up prices. In short, choose less and choose better.

Versatility is possible even in evening wear. A blue satin blazer can run the gamut of just about every occasion but the office or a football game, says one designer. The same is true of the black mohair suit; that can even to to the office set off with, say, a regimental tie.