The way to do this is to first open the older version of the part, then open the assembly - it will use the part in session rather than the latest version.
If you are using Intralink you can do a search that allows you to find all the versions of a part. Check-out the older version you want. Be sure you don't have anything else in your Pro-E session (memory). Then bring the old version part up in session first. After that you can bring up your assembly model and the older version of the part will be in your assembly. Of course this is how I'd do it using WF2.0 and Intralink 3.4.
Im pretty sure that, everytime you save a part in pro/e, it'll save it as .prt.1, .prt.2, etc... So as SrGillberto said, make sure your Pro/E session is cleared, and find the one you want. Save it. That'll will beomce the most recent save. When you open the assembly, pro/e will take the .prt saved most recent, hence the .prt.1 and so on.