Ya there are chances of that happening. I have noticed lots of people sobbing because of that. Its always sen that after changing of host for some time there is a marked decreased in Adsense earning too. I think this might be dut to the reduced traffic to the site.
But exact change in
PR is a bit rare situation.I think if you intend to do that its better you do it now so till the next
PR update you can again set up a proper base for your blog.