To me, that sounds like, uninstall the current client (12.1 base) with the latest client version (12.1 RU1 MP1).
If you are not looking to upgrade the SEPM server immediately, but rather want only to update the clients (for whatever reason), I.E, they are standalone, than the answer is YES.
You can directly migrate from 12.1 to 12.1 RU1 MP1.
I haven't tried it myself, but according to this article from Symantec, you shoudl be able to.
http://www.symantec.com/business/support/index?page=content&id=TECH187658
"Please find the Symantec Endpoint Protection 12.1 RU1 MP1 Client-only patches for Symantec Endpoint Protection 12.1 in the Attachments section.
Release Date: April 27, 2012
Description: This .zip file contains patches to upgrade Symantec Endpoint Protection 12.1 32-bit and 64-bit clients to version 12.1 RU1 MP1 (12.1.1100)."