If you have an R77.30 environment - MDS plus an SME server, it is up to you to decide which server you upgrade first.
You can upgrade the MDS first and leave the SME for a later date - the SME will not be fully functional during this transition period, but it won't impact your MDS server functionality.
I guess the downside is that your SME server is only partially functional - I wouldn't go as far as saying it's in an unsupported state, as it's supported. But the 'support' for any problem will be "Please upgrade your SME server".
This is the same situation for your MLM - a log server has to be in the same version of your management server, but it is an acceptable upgrade approach to start with your MDS server, test it, and then proceed with upgrading the rest of your environment.
It's an unstable but supported state. The behavior during the transition period will be unstable, but once you finish upgrading the rest of your environment things should go back to normal.
The main difference between MLM and SME is that your MLM should always be in the same version as your management while the SME also supports forward compatibility (since customers often, for various reasons, upgrade the SME before the rest of the environment).
HTH
Yonatan