May be a trick question, but would it be when Alan Moore took the series over in 20 but really started his take on the character as a creature versus a prior human being in issue 21?
Also, very interesting entry under wiki concerning this question:
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In issue #6, editor Len Wein declared, in response to a published letter, that Alec never had a brother and that every Swamp Thing series story after issue #21 of the original series never happened. The letter, however, questioned why Swamp Thing had reverted, which had already been explained in the Challengers of the Unknown run. A later column pointed this out, so they said they would not deliberately contradict it, even though they would still go from the assumption that it never happened.
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