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No, you don't have to completely reinstall the game. Just install it over you 1.05 patched game
Save games might work, but might also not work. Paradox never gives guarantees about this. But since there aren't changes made to the .exe file in the IP, continueing a saved game shouldn't cause any problems. It only might be that you get duchies, kingdoms or cultures in weird places (because of the tag-changes)
If you want to continue playing with 1.05, but also want to try out the IP just install the game twice (or more times) on your PC. f.e. rename your current CK folder to: CK patch 1.05 and then do a complete reinstall and install patch 1.05 and the IP on top of that