Without having used Vantage yet personally, it's hard to say for sure.. I imagine it would mostly seem ok, but there could be some issues, like functions or fields from Vantage which may not have existed in ClassiPress which would then involve some manual input. I am thinking back to when we upgraded from ClassiPress version 2.9.x to 3.0 and how certain features didn't automatically work for legacy ads..
My guess is that it could be a bit of a hassel depending on how many ads you have on your site at the time of changing the theme.. Personally I would think if you were going to go to the trouble of customizing ClassiPress, you may as well make it how you want and stick with it, or hold off altogether until Vantage is released. That's really your call though of course..
