I figure a few of you might have more questions about faction switch than they covered in the FAQ. Feel free to post if you've got any info I miss.
Pet names: Your demons' names stay the same. This could have some fun uses for sharing your demon's name or something.
Quests: I haven't looked very far into this, but you may be able to repeat some quests (including class quests). The quest to learn VW was there at the class trainer even though I've got mine. Although a non class quest I tried said "you have already completed this quest" when I tried to accept it. So I'm not sure what extent of all quests can be redone.
Map: This isn't specific to locks, my map only shows areas I'd already explored. My new faction still had the major cities unexplored. I switched at level 72 and so far I've got all the Kal/EK and Outland flight paths. I haven't checked Northrend yet.