From what I gather, it was a gradual shift that began in the early 80s but didn’t really become the norm until the end of the decade. https://www.boxingforum24.com/threads/top-80s-fighters-last-same-day-weigh-ins.589623/
They were staggered, according to which sanctioning body was involved. By the early 90s, I believe, they'd all made the move. I distinctly recall one of the Tyson-Ruddock cards in 1991, the weigh in was televised on ESPN the day before, which seemed strange. It was certainly not in 1980, which some in the link above stated. Most major fights still held weigh-ins the morning of the fight throughout the decade of the 80s. All the Leonard, Hagler, Hearns, Duran fights. I even recall boxing magazines doing profiles of boxers - following them on the day of a fight - and I remember one article featuring Alex Stewart in 1988 or 1989. And he weighed in for a fight in New York late in the afternoon ... and the fight was in a couple hours. In the book The Black Lights, Thomas Hauser talks about how Billy Costello spent most of the night before his defense against Mamby trying to lose three pounds before the morning of the fight weigh-in. That was in 1984. In the link, someone said he thought Holmes-Zanon weighed in the day before their title fight. That may be true, but Holmes-Zanon took place on ABC early Sunday afternoon. And, as I recall, the main event started at noon in Nevada (3 p.m. on the east coast), and the undercard began at like 9 a.m. in Nevada. So they weren't going to weigh in that morning.
