If you want to move from a 32-bit version of Windows to a 64-bit version of Windows 7 or vice versa, you'll need to back up your files and choose the Custom option during Windows 7 installation. Then, you'll need to restore your files and reinstall your programs.
Re-install is the only way I know of. However the virtual machine route (like VirtualBox) works well for me for running older 16bit software. As you do not give an indication of the software you want to run we can't say if this is your best choice.