I started an campain before the patch 1.6 and was able to continue with it. But the balance of cities may have changed, so check your empire. As after the patch the Academy also helps to keep a city in order, I was almost able to continue after I patched with nearly no trouble.

My answer is related to a BI campain running the Sassanids. I have no idea how much it affects a regular RTW campain. I fear that non-roman fractions will get in hot water after the update, because only romans can still arrange games in the colusseums. It will be hard to keep order without them after you took roman cities.