I'm afraid when you change your url, the PR will be reset. Unless of course you manually ask each link to be changed which is near impossible
You will still keep the PR for the blogspot URL which still works. On the next update, you will see that they match. Both URLs of mine are the same PR even though I only promote the custom domain.
I think it is really difficult. How many posts have you published? But I know one blog have been changed from blogspot to normal domain. The webmaster is Rachel, who is also a DP member. Her blog is turnonepoundintoonemillion.com. Hope this will help you...