Yes and No, you cant just swap the 16GB flash memory out for the 32GB one, as it is attached to the motherboard (or "logic" board as the Fanbois call it)
However If you were to get a 32GB or even 64GB "Logic" board you would be able to swap them out and have a bigger memory
my only concern is you would have to make it match your phone completely, for example a iPhone 3G board wont work on a 3GS and vice versa, or 4S wont work on a 4g and vice versa.
Also, I personally haven't tried this yet so I dont know any potential problems or side effects there may be.