Given that we bounced back under Warnock in spectacular style, I’d go for later than the mid-80s (though that was a wretched time with the consecutive relegations). Turning five defeats into victories would have kept us up in 1994-95, a season where we had more managers than away victories.
But if I could change one result in our entire history, it would be the 2-1 Boxing Day defeat to Macclesfield in 2018. If we win that, we end up on 44 points and Macc. on 41; we stay in the league and they go out. God, we were diabolical that day.