Hello,
What version of SEP 12.1 are you presently carrying on your machines?
Incase, if you are carrying the SEP 12.1.671 (RTM Release), then Yes, To migrate to SEP 12.1 RU1 MP1, you would require to Install SEP 12.1 RU1 first.
Check this Article:
Upgrading or migrating to Symantec Endpoint Protection 12.1.1101 (RU1 MP1)
http://www.symantec.com/docs/TECH187753
NOTE: About Migration paths
Symantec Endpoint Protection 12.1.1101.401 (RU1 MP1) can migrate seamlessly over the following:
Symantec Endpoint Protection 12.1.1000.157 (RU1)
Hope that helps!!