Short answer - yes.
But, there could be elements used in the earlier Project, that do not appear in PrPro CS 6, so that could be a concern. The more "plain vanilla" that CS 5.5 Project, the greater the likelihood that it will convert and Open in CS 6. Now, going backward might not work so well, so do NOT overwrite that CS 5.5 PRPROJ file.
Good luck, and if concerned about Effects, etc., let us know what has been done in that CS 5.5 Project.
there could be elements used in the earlier Project, that do not appear in PrPro CS 6,
Such as? Going from 6 to 5.5 might be an issue, if Adjustment Layers are used. But I'm not aware of anything in 5.5 that was dropped for 6, so even the most complex 5.5 projects should open just fine in 6.
I have several projects done in the past and a few that are in progress. I'm assuming that it's best to finish all projects before upgrading?
I use various color correction filters native to CS5.5 as well as Red Giant plug-ins (looks, denoiser2, colorista mostly).
>I'm assuming that it's best to finish all projects before upgrading?
That is always a good idea... also, be aware that CS6 has a Bug with AVCHD http://forums.adobe.com/thread/1004369?tstart=0
Some comments in http://forums.adobe.com/thread/1205851 indicate that the bug MAY be fixed... but no promises
If you don't work with spanned AVCHD files, you don't need to worry about the bug