1978, possibly 1975
My own research into the two oldest modules I own revealed (both AD&D modules published by TSR in 1978) revealed two cases of variant monsters under the revised definition.
First, Area 29 of the upper level of Module G2 Glacial Rift of the Frost Giant Jarl features a black pudding that, while conforming to all normal black pudding stats, is white in color to blend with the snowy, icy environment.
The second is Area 8 of S1 Tomb of Horrors which has a "mutated" gargoyle with four arms and correspondingly more attacks. This module also has an animated hill giant skeleton, though that could be defined as a normal hill giant under the effects of a specific spell (animate dead), so it doesn't qualify under the revised definition.
Since Tomb of Horrors was first designed and played at the 1975 Origins Convention, it is possible thatitthat it was published in a limited run that year. I know there were tournement prints of C1 Hidden Shrine of Tamoachan sold at the 1979 Origins Convention that are now collectors items, but I cannot confirm whether this was done in 1975 with Tomb of Horrors. Without multiple copies made available to the public, it wouldn’t count as published. Further, the Tomb of Horrors Wikipedia page states that the adventure was revised for publication in 1978, so I cannot confirm the mutated gargoyle was in the original.