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Mrs. Krabappel WILL be written off "The Simpsons" in the wake of the death of the actress who voiced the character ... but it's not going to happen for a while, TMZ has learned.

TMZ spoke with "Simpsons" E.P. Al Jean who tells us ... no one could possibly fill Marcia Wallace's shoes as the voice of Ned Flanders' 2nd wife -- so they're not even going to try.

Al says producers do plan to write Edna off the show -- but there's no rush, because the series is produced so far in advance, Marcia had a year's worth of episodes recorded when she passed away.

TMZ broke the story ... Edna is NOT the mystery character that producers had planned to kill off the show this coming season. Al wouldn't say how Edna will leave Springfield.

As for Marcia, we're told the legendary comedy actress will be laid to rest in the next few days in a private ceremony.