What is the process on returning to your previous MOS?

It’s called a reclass without training. You will need a memo from the proponent of your old MOS stating you are still qualified in your MOS.
After that, HRC decides which MOS they need you in more. Once you get the memo talk to your Career Counselor to process the reclass

During deployment, no. You are MTOEd into a position based on rank and MOS. If you have a secondary MOS, you will need your current branch manager to release you to your other branch