Currently, the documention for the migration paths suggests SEP12.1RU2MP1 can only seamlessly be installed over SEP12.1RU2 as per:
http://www.symantec.com/docs/TECH204449
Note: The article does not differentiate between the SEPM upgrade and the Client side upgrade paths.
Despite this, we can clearly see some of the fixes in SEP12.1RU2MP1 directly relate to performing client upgrades from SEP clients older than RU2:
http://www.symantec.com/docs/TECH204685
So to that end, I've created an IDEA for Symantec to document and support more upgrade paths of SEP Clients to the latest version. Please let me know your thoughts, and add your voice to the IDEA if you believe it to be of benefit:
https://www-secure.symantec.com/connect/ideas/support-more-sep-121-client-migration-paths