The general upgrade process here is to switch the libraries to the current version's and load the subset library for the previous version.
We also have a company library for each version. When a new version comes out, I copy the entire library, flag anything I don't want people to use anymore as "not placeable", and add/revise objects for the new features.
I've got a couple AC8 jobs that were brought all the way up to AC11 that way and they usually work. The problems usually came from reference markers, since the new markers work WAY differently than the one ones. (On a side not, once I started making my own markers, that problem went away)
Tom Waltz