Even though I have posted a whole series of articles about that upgrade process, there is one small but important detail to reckon with. Otherwise OM12 will generate errors in the OpsMgr event log of the OM12 Management Servers.
It’s all about the OpsMgr and OpsMgrDW databases which still run in the SQL 2005 compatibility mode. OM12 doesn’t like that and will throw errors in the OpsMgr event log.
KB2700028 tells you how to solve it.