Maybe you did not read my post, you can certainly start 64 bit apps from a 32 bit client - therefore you do need to not change it, but you dont get the community features that steam provides. Maybe it could be done with a 64 bit overlay that the app itself preloads in that case. Install World of Goo and Amnesia and see yourself what i mean. I highly doubt that there is one game out here that runs faster when using 64 bit but some games have got sound issues. With direct pulse audio support this should be easy to fix however.