No need to change anything.
Only if you have a program explicitly designed for 64 bit( There are very few of these. )
would you have a problem running it on a 32 bit os.
If, for whatever reason, you wanted to change vista from 32 bit to 64 bit, you would have to do a clean install of the 64 bit version of the os. You can use the oem key that you already have on each 32 bit machine.