The fight was originally set to go down on Sept. 12 -- but now, we're told the new date is locked in.

So, why the change to Thanksgiving weekend?

"Changing the date to November 28th will give more people the opportunity to see the biggest comeback in boxing history," Tyson said in a statement.

"This temporary inconvenience will last longer than Roy Jones Jr. He better be ready, I'm coming full force."

The company behind the event, Triller, says the move was made to accommodate a larger audience than originally anticipated.

"Given the enormous interest, the holiday weekend will make this historic battle an even bigger viewing event," the company says ... "marking the first live Pay-Per-View event brought to the world by Triller as the first of the 'Triller Battles' series."

Triller is also officially announcing the addition of new undercard fights ... including Badou Jack vs. Blake McKernan.

Badou Jack is one of Floyd Mayweather's fighters -- he's 22-3-3 and a former world champ in 2 weight classes.

Jack says he's donating a portion of his fight check to the Badou Jack Foundation to "continue to give kids across the globe a fighting chance in life."

Also added ... Viddal Riley vs. Rashad Coulter.

Vidal is pretty talented British fighter who's also famous for training up KSI for his fight with Logan Paul.

Coulter is a former MMA fighter who competed in the UFC until 2018.

And, of course, Jake Paul is still set to fight ex-NBA star Nate Robinson as the lead-in to the main event.

We're told the Tyson vs. Jones fight -- the first event in a series under Mike's "Legends Only League" -- will be sanctioned and scored by a recognized sanctioning body. 

 Oh, and the winner will get a specially-commissioned belt!!!