You have a bit of an either/or choice here (at least if your libraries are like the ones we have here in the states).
The AC10 (converted) project can load the AC9 library and all should be fine. Over here, if we load the AC10 library instead of the AC9 library it will recognize most of the parts. Even though the names have changed it recognizes the internal IDs.
I generally prefer to use the AC10 library where possible since it is notable improved over the AC9 version. This sometimes means redefining a few unrecognized parts - ones where the AC9 item doesn't have an AC10 counterpart (I have so far had no trouble finding a substitute).
Loading both libraries is not a good option (at least in our case) since it creates a long list of duplicates and I believe there are significant functional differences between the versions.