Speaking of plains to forest, I never do this. I'll change jungle to grassland and when I can, I'll change plains to grassland unless the city has really poor shield production. By endgame, I try to switch everything to grassland with farm, essentially, as much food production as I can. So when it ends, all cities are maxed out in population. But that's just me. Rather than build more cities, I'll wait until all land is maxed out for all currently exsisting cities. Call me an anal perfectionist.