I got windows 7 32 bit when my windows vista home premium was having problems and i contacted them for support. They no longer supported vista so they upgraded my computer to windows 7 32 bit for me. (i didn't have to pay for it)
i was having problems so i went to the windows vista help center on website they said windows vista help was no longer supported and upgraded me to windows 32bit because it had many problems. i dont particularly understand why they upgraded it because i was okay with windows vista.
If you have a legitimate product key they are interchangeable between 32 bit and 64 bit (as long as it is the same version ie. Home, Professional, Ultimate) so you would just need to get access to a 64 bit install media and your key should work fine -- That said Who upgraded you ?? was it MS or some third party company - If third party I would worry that the the copy they gave you for no cost was not genuine and therefore you might have problems with it not passing activation when changing to 64 bit.