Taking over abandoned sub-sites

What can I as the network admin do with abandoned sub-sites? If a user decides they want to cancel their subscription and abandon their site is there an easy way for me to take over their site (with their permission of course).

How would that work with Pro Sites levels (I'm not incorporating a free level by the way). Do I just leave it at the same level and pay for it myself, then change then make myself the admin.

Any feedback would be appreciated.