how?

DNS will resolve to posterous' servers. It's out of your old server's control by that point.

Unless you're talking about different domains, e.g. moving from

mycoolblog.com self hosted -> myawesomeblog.com on posterous

in that case you could do what you said, but I don't think that's what the OP is talking about. the issue is when you use the same domain on both.