What is the first known example of published expert chess analysis that was disproven by a computer search, with the winning player completely opposite? That is to say, a widely known published analysis by a chess grandmaster of a position with the conclusion that it was winning for one player, but later a computer search proved that actually it was winning for the other player. This comes close, but I am curious to see examples with win-to-lose reversal of opinion. To clarify, I am looking for the first time we discovered that the previously accepted analysis was wrong, and our discovery arose from a computer search.
In the early eighties, the evaluation of the two bishops vs knight endgame was changed thanks to computer-assisted analysis.
Previously, that endgame was thought to be a draw, but now we know the bishops can force a win.
I don't know if this is the earliest instance, but it's definiely a quite old one
Partial answers because this is an intrinsically difficult question, especially to prove it was the "first".
Queen versus two bishops. This was thought to be a draw due to the existence of a drawing fortress position, but the queen can win most of the time by preventing the bishops from getting to the fortress. However, it can take up to 71 moves to force a win (Nunn 2002:290ff).
The question also asks about grandmaster analysis being incorrect, with the actual result being the opposite. This certainly has happened before simply because grandmasters are fallible humans. Example by Tim Krabbe:
3r2k1/4qpbp/Q4P2/8/2N5/4p3/PP4PP/5RK1 w - - 0 1
Black to move. With the powerful passed pawn on e3, Black is clearly winning. 26...e2 27. fxe7 Bd4+ and White Resigns.
This game was published as a brilliancy, and it was months before someone spotted the fatal flaw in the combination: the position after 28...e2?? is actually winning for White. I won't spoil it here, but if you're interested you can find the solution on Tim Krabbe's website. Of course, in today's world, errors like this will never get published because an engine will spot the winning move in a fraction of a second.