Hello Bonner,
YES the manager would still be able to Manage MR5 clients and will be able to do the basic stuff like policies virus defs , logs forward.
the technology of communicating with manager is remained same. Not many things changed.
Your previous manager can still manaintain SEP MR5 clients
to test this you can install a clients MR5 as unmanaged ( as your manager is not mr5)
take the sylink file from manager as per this document, the client should have green dot and should be reporting to the MR4 Manager
How to change a Symantec Endpoint Protection client from unmanaged to managed holds good for all versions
http://service1.symantec.com/support/ent-security.nsf/docid/2007082009543848