As Trouble mentions, you cannot "upgrade", meaning an in place upgrade, from a 32 bit to a 64 bit version, they are not compatible. You can do a clean install of the x64, which would wipe the old install.
The x64 version does allow the use of more memory and a couple of other advantages, but to me the biggest drawback is the x64 drivers are not as available as 32 bit. If you have older devices that might need drivers, you may want to check that prior to upgrading. For instance, I have an older HP scanner that does not have a x64 driver and will not work in that version of Win 7.