Im from Finland so i havent got my civ4 yet,
anyways
I read that when you get some religion tech your city will turn to Holy city of that religion. Or do i get holy city when i declare state religion? And if i change my state religion what happens to holy cities from different religion that are under my control, do they keep spreading that old religion?

I read that when you get some religion tech your city will turn to Holy city of that religion. Or do i get holy city when i declare state religion? And if i change my state religion what happens to holy cities from different religion that are under my control, do they keep spreading that old religion?