Hello Johan, and welcome to Seven Forums.
Q1) The product key (license) can be used to activate either (not both) the 32-bit or 64-bit version of Windows 7 Pro, but only on that exact same PC the key was originally activated on.
Q2) No. Once an OEM key has been activated on a PC. It is permanently tied to that specific PC and it's motherboard. It will not activate on any other PC. There's no need to worry about the product key since you can still use it as you wanted with Q1.
Q3) You could just do a normal clean install
of the 64-bit Windows 7 Pro on the same PC that you activated with 32-bit, and activate it with the same product key.
Hope this helps,