It depends.

If nothing changes about your flight from now until you fly, chances are you'll keep your seat.

If there's a plane type subsitution and everyone's seats get reassigned, it's a bit unclear what would happen: Would they still put you in MCE because that's what you booked, or would they look at your new status and say "does not qualify for free MCE any longer"?

There's no good parallels in the past, because even if you qualified for a free-in-advance exit row seat (the only type of "better" seat available before MCE), and even if you had said online that you were ok with an exit row seat and that you met all the FAA requirements for sitting in an exit row seat, if they changed planes they would automatically dump you out of the exit row seat. But that could be because of FAA requirements, and it does not explain how things would work now in non-exit-row MCE seats upon plane substitution.

(I presume you have MCE seats that aren't exit row seats? I have to ask, because on many planes the exit rows are part of MCE. But how things get handled upon plane substituion might be different betwen exit-row MCE and non-exit-row MCE.)