I think I found the answer. According to Microsoft documentation (below), it appears that subscriptions will continue to work after SSRS 2005 is upgraded to 2008. However, I have not tested this yet.
Use this link and at the bottom it has upgrading reports link and this one has breaking and behavior changes links. None of them say anything about subscriptions not working.
http://msdn.microsoft.com/en-us/library/ms143747.aspx