As 32 bits e Perguntas frequentes sobre o Windows de 64 bits afirma:
If you want to move from a 32-bit version of Windows to a 64-bit version of Windows Vista or vice versa, you'll need to back up your files and then choose the Custom option during Windows Vista installation. Then, you'll need to restore your files and reinstall your programs.
Assim, faça um backup se você quiser e depois reinstalar, não há outra maneira possível ...