You can not upgrade from 32-bit Windows (any version) to 64-bit Windows 7. You must perform a clean install (the operating system is installed from scratch with the entire hard drive being wiped). This is because the 64-bit operating system uses an extremely powerful and complex method to make 32-bit applications/programs work nearly seamlessly in the 64-bit operating environment. In that environment, nearly all Windows core system files are 64-bit, and the operating system is innately incompatible with a 32-bit operating system upgrade.
All full retail versions of Windows 7 contain both 32-bit and 64-bit versions except Windows 7 Starter edition, which is only available in emerging markets. If you have an upgrade disc for Windows 7, you may not qualify for a Windows 7 64-bit full edition disc, and may have to buy the full edition.
Here is some good news:
If you have a OEM product key and its on the back of your computer, or if you have the full edition of Windows 7, your product key/license is generally known to work for both 32-bit and 64-bit. Unless this policy has changed recently, all you need is the 64-bit disc. You may consider contacting Microsoft and asking them for an upgrade to the 64-bit version, indicating to them that you have a valid license. Check and see if they will send you the 64-bit install media. During the initial launch, there were many reports that it was possible to obtain the 64-bit version for a relatively small fee.
Remember, however, if you only have an "upgrade" edition of Windows 7, it will always be 32-bit, because you can't upgrade a 32-bit operating system to 64-bit. The architecture is incredibly different.
Be aware of the RAM limitations in each edition of Windows before you move forward. Home Premium can only utilize up to 16GB of RAM while Professional and above can use up to 192GB of RAM.
Professional and above have better encryption, remote desktop, group policy, networking, and Windows Server support.
Every copy of Windows full edition is essentially the same, except features are locked based on the edition you have purchased. "Upgrade-only" versions of Windows are only capable of 32-bit upgrades, because you cannot upgrade from 32-bit to 64-bit.
Full edition retail boxed versions of Windows 7 contain the 32-bit and 64-bit version.