2nd-3rd went south

I have a small block 727 from a gts 340, 2nd to 3rd gear shift went "buttery" I was told by the previous owner, and I have never done more than a torque converter change, so I have no idea what the issue is, nor how to fix, I read alot, though, and I began to suspect a valve body change would fix, but I don't know anything about bands slipping or all sorts of other arcane formulations that I see here aand there in various publications...So since I am an absolute don't-know in this regard, is there some one out there willing to clue me in, and also inform me if you could about the viability of a four-speed conversion? Since I am finishing, well, completely changing someone elses project this is the time to make big changes, and any advice will be gold to me!! Thanks... :dontknow: