I was wondering if someone could point me to the files to change to make old culture easier to shift. I mean taking a city in the late game the old culture is very hard to remove. Can anyone help me alter this ???

just drop a spy or two and seed your own culture first then take the city using force.

IIRC a spy only adds about 5% culture. So you need to put 10 or so in there to get things on your side

No I meant the culture on the land surrounding the city many turns after it has been taken.

Is there an XML file for this ?