I don't think a page rank will decrease just because of the ownership change. It has nothing to do with it (or am I missing something important here?), it has everything to do with backlinks. And if they're still there, it won't decrease.
Anyway, keep in mind that page rank is not the most important factor for rankings, I've seen websites with page rank 2, outranking page rank 4's for relevant keywords. Don't just buy a crappy domain for it's page rank. Traffic is also different for any "page rank" domain. Link driven traffic is different, number of ranking keywords achieved and it's search volumes etc...
You could buy a 6 month old domain name with 0 page rank and build it to a page rank of 4 just in one month. I know when I registered my personal domain
www.aleksandar.eu, some of the people had linked to me and I had a page rank 4 within a month or so. Now it's 2 or something, some websites that linked to me are down or changed they're designs and forgot to put back a link. I'm not complaining it's an empty url anyway... But the point is that it's page rank would've remain the same if the same backlinks were present today. Now, there's one little problem, I don't know for what keywords I would rank high with a page rank 5 considering it was an empty site.. :smilewinkgrin::lol: