First of all the existing "Rewatched" is not the same as what I'm requesting, as it tracks completed Rewatched shows.
If I mark something as "Rewatching", it should move the show into currently watching, state that it's being currently being rewatched, and upon completion +1 the "Rewatched" count and move it into "Completed".
Once stats come back, the distinction between "Rewatched" and "Rewatching" will be even more important as it I'll mess with total watch time. If I start rewatching FMAB and I'm only completed the first episode, I don't want my stats to say I've completed the whole thing again.