Quote from Swervish
PLEASE PLEASE PLEASE tell me there is a mod that will raise the upper sky limit about 100 blocks. I'll, by force, have to use one mod.
I haven't used his mod and am not sure if it works on previously generated maps. Wost case scenario you could make a new map and use mcedit to copy all your progress so far into a new map. If you don't want to use mcedit then you'll have to start from scratch. Sorry for the rough news.