Under the EULA you are entitled to use 32- or 64- bit Windows 7 with any license, and no manufacturer can restrict this.
Some don't make 32 bit drivers
available though they're almost all available in the installer, via optional Windows Updates or searching on the web.
When you clean reinstall Windows 7 with the Product Key on your COA sticker you change the activation from manufacturer to the control of MS Activation servers so your manufacturer has no say in this beyond that point anyway. They may not tech support the clean reinstall (don't tell them) but it cannot void your hardware warranty.
In your case the problem is that you must boot the installer to change from 32 bit to 64 bit or vice versa, and cannot run the installer from the OS. A proper clean reinstall should be booted, and you need the DVD or flash stick installer for repairs anyway.
Here is the latest official Windows 7 ISO download: Win7 Home Premium x64 SP1 English Official ISO download.
Burn to DVD or write to flash stick using Windows 7 USB-DVD Download Tool
Then follow these steps to get a perfect clean reinstall to the Windows 7 partition: Reinstalling Windows 7
. Afterwards install EasyBCD 2.02 to add XP to a new Dual Boot menu.
You may need to make the quick robocall to activate to record your hardware signature in MS Activation servers.