Just a quick comparison of two strips that appeared in one of the few Pogo collections I own (at least until they get that Fantagraphics collection off the ground, but that just never seems to happen). One is from 1949 and the other from 1958. They both use the same gag, which isn't unusual, but it provides a chance to look at how the character designs and other aspects of the artwork evolved over the first decade of the strip. Obviously it is more the same than it is different, but the characters have all gotten a little cuter.
Oddly enough I think the most noticeable change is the lettering. Originally it was pretty simple, and over the years Kelly (or whoever was lettering for him by then) started EMPHASIZING certain WORDS in order to CATCH THE READER'S ATTENTION.
Porkypine smiles in 1949:
Porkypine smiles in 1958: