You can update the design on the resource database and the 5.x clients will have no problems. Notes templates are designed to be backward compatible. You can also rename resources and all existing reserverations will be updated. You need to rename them from the resource database, not from the Domino Directory.
Markus - what was the bug you encountered please? Are you referring to server or client versions (6.5.1)?
I need to upgrade the design of our Resources database, so we can rename a resource. Our servers are 6.5.4. But Iโd be interested to know what problems you ran into first!