the only problem i have with culture is that it forces u to keep fighting to get a decent amount of workable tiles in a captured city. Let's assume u capture an enemy city as early as 500 AD. u have enemy cities surrounding that city with refined 1000 culture points. that leaves u how many workable tiles - 3? 4? and that's 500 AD!!! at 1500 AD u'll be lucky if u're left with 1 tile (besides the city i mean). let's assume u then stop fighting with that civ for the duration of the game. peacefully, u'll never be able to produce enough culture to match the enemy cities - so u're stuck with an unworkable city - u have no choice but to keep fighting - that sucks.
I agree that conquering an enemy's capital with legendary culture then immidietly switching it all to the conquerer would make warmongering way too powerful. how about a middle way?
i suggest 2 things that go hand in hand:
1) the old culture will not fully return in the beginning - it will gradually pick up - starting with 0% up till 50% (i still think 100%) in a span of about 50 turns. the reasoning is that after that 50 turns, that city will peacefully be able to be workable.
not to ruin cultural victory, put an upper limit on the amount of culture that can be gained, or else, make "conqured culture" not count for cultual victory.
2) during those 50 turns, increase the riots probability in a city a lot more than what is present now - make it genuinely hard to keep a conquered city - that will force warmongers to station a lot of troops there for a long duration of time.
i don't know the formula, but the riots probability should be linked to the population a city has, the culture borders of ur enemy civ, but also to the amount of culture the city had as well (it should also be linked to the relation the captured civ has to ur civ but i digress).