There are too many changes to list, but here are a few:
Straw boater hat was popular in the 20's, they are much harder to find in the 40's.
These "plus four" pants were worn as casual pants in the 20's.
A variety of starched collars were worn in the 20's, in the 40's most shirt collars are the standard ones you would find today.
Suit jackets were a little more colorful in the 20's, in the 40's they became duller and boxier looking with very wide shoulders and lapels.
etc.
The silhouette definitely changed!
We came out of the Great War with the narrow Edwardian silhouette, with a high stiff collar of several styles. These buttoned to the collar band with shirt studs and were removed for laundering.
The next major steps were the widening of trouser legs, the Oxford Bag. In the extreme, you could accidentally put both feet down one trouser leg. At the sametime, the jackets got very wasp-waisted. You can see this in the cartoons of John Held, Jr. More men took up a shirt worn in the military, the "soft collar" sewn to the shirt, especially in work shirts and athletic wear. Ties came down to the bow tie and four-in-hand, while ascots and other styles faded away. The straw boater was only summer wear. The bowler began to look old-fashioned.
Going into the Thirties, legs narrowed but lapels widened and jacket bodies loosened. Like women's fashions, this oscillated slightly, but didn't much change until well after WW2.
Of course, we also got the minority fashion for the zoot suit: tightly cuffed peg-top trousers w/very high waist, a knee-length jacket with very wide shoulders and lapels, with a very broad-brimmed hat.
Edit: adding links
