upgrade an ipod touch's flash drive?

no, not without swapping out the whole logic board, take a look at the teardown photos, e.g. Step 13

iPod Touch 3rd Generation Teardown

the boards are different because of different flash memory chips. but even if the boards were the same and the 32GB version simply had more memory chips installed than the 8GB, it's unlikely you could solder the parts at home by yourself with an ordinary soldering iron. these kind of circuit boards are usually IR or wave soldered by machine, not hand soldered.