Ben,
TGC has redone all of the model packs (except for maybe model pack 1) to work with the latest version of FPSC.
So at the very least if you have model packs 2-5 installed already you would want to uninstall the old versions before installing the new ones.
What I had to do was uninstall RC9,then I had to uninstall the v1.03 update (for some reason when I installed anything from RC2 on up it didn't overwrite the original V1.03 file,explaining at least some of the problems I have had) and uninstall all of the model packs.
Then I installed RC10 and all six of the model packs.
While none of the .fpm files I have previously made will run properly in RC10 I started a brand new level and everything is running fine up to this point.
In case anybody is wondering I have an Emachines computer,an ancient Intel graphics card and a single core processor.