Got a solution for ya then. I wanted to just keep a few buildings but not the whole map, because of the new biomes/ores. So i just converted the ID's for redpower, because that was the only type of mod block i'd used in my buildings. Then i used the latest version of MCedit to copy the building across.
Happy to say it worked perfectly, which is a relief since i've been trying for ages now. Posted a copy of the ID conversions for redpower to get you started. Hope it helps. Also, don't rotate red power, as it screws up the orientation for its blocks. Just straight copy paste.
NOTE: I had first upgraded my tekkit classic map to tekkit lite which i found
here
The ID's do change from tekkit classic to tekkit lite, and my conversion is from tekkit lite