I'd like to know this as well. The only thing I've been able to find so far is this article:
But I'm not sure which parts are relevant to 5.1 and it doesn't talk about SSO, inventory, and web client services. Step 5 concerns me, "If you are migrating a vCenter Server 4.x/5.x database, you must also migrate the ADAM database. You can migrate this database if the Windows versions in the source and the destination is the same. If not, vCenter might not operate properly and/or the client might fail during connection"