i don't think he need move personally. If HBK isn't swapping shows then Taker won't either. They want one of their two legends on each show, not both. I personally am waiting for Taker to turn heel. It's been a good while since we had a heel Taker and his promos are a lot better than when he's face (well let's face it, he barely ever speaks when he's a face), however we're more likely to see HHH turn heel than Taker.
There are only 2 people on Raw that Taker could fued with and that's HBK again or Jericho who apparently he has never wrestled.
I could see one of two scenarios playing out.
1.) Vickie moves Undertaker to Raw because she doesn't wanna deal with him anymore. He faces HBK at Wrestlemania 25.
2.) He stays on Smackdown and wins the Royal Rumble despite Vickie stacking the odds against him and he faces a heel Triple H at Wrestlemania.
In scenario one she hires help like Big Show or Edge to take out Undertaker. Big Show could work because he wants to show Vickie he deserves a WWE title match and not to be overlooked. Edge would require the feud between him and taker to continue but it would get him in Vickie's good graces again if he doesnt feud with Taker again he might have to go back to Raw. What if Adamle trades Randy Orton for Edge because Orton has unfinished buisiness with Triple H and Edge needs to get away from Vickie. Edge staying on Smackdown would require a last ride match between Undertaker and Edge maybe. I don't think they'd have a casket match or a Buried Alive match with all the deaths recently and Vince wanting to stay PG. But a Hell in a cell match is a much better finish to a feud than a Last Ride Match. The help doesnt work so she just ships him to Raw in time to setup a feud at the Royal Rumble with HBK for a eventual match at Wrestlemania.
In scenario two the beginning of the first scenario plays out but instead of her sending him to Raw she uses the Game Triple H who is heel at this point and they have a Wrestlemania showdown where Undertaker defeats Triple H and wins the WWE title. Prevailing over Vickie Guerrero.