My guess is that the percentage of players who completely quit CH to go to WH:O is going to be quite small. Yes, I'm sure quite a few people will want to check it out, but I don't think it's going to be as massive of a move as you seem to be expecting. The two games do seem like they are going to be quite different after all, and WH:O may not appeal as much to CH players - if not in lore and storyline aspect, than perhaps in just general gameplay.
As for cross game transfers - those are not allowed, so please don't bring them up here on the forums.
Not sure, when Nuada began it had a huge flood of players. Obviously people are going to play the new game just to try it out too, new is exciting. Especially when there's hardly anything "New" for long periods of time.
For me I would feel excited because I get to be the first group of people to play the game and eventually become once again one of those cool people who say "Oh I was here day 1".
Did I mention its going to be completely full of new content?
If they make it dramatically similar to Celtic Heroes gameplay, people might move just because they're bored to death. I sure know I am.