The goaltending showdown between Theodore and Montreal's David Aebischer, who were traded for each other in March, turned into a shooting gallery with goals in bunches from both teams, with Colorado outshooting Montreal 38-36.
Theodore, greeted with derisive chants from the Bell Centre crowd of 21,273 throughout the match, was beaten by Michael Ryder during a five-on-three 29 seconds into the third period, then saw the floodgates open for goals by Craig Rivet, Chris Higgins, Alex Kovalev and Mike Johnson.
Sheldon Souray got his fourth and fifth power-play goals of the season and added two assists, while Sergei Samsonov also scored for Montreal (4-1-2).
Colorado had two goals from Andrew Brunette and singles from Joe Sakic, Tyler Arnason and Paul Stastny, the son of former Quebec Nordiques star Peter Stastny who scored his first NHL goal.
It was the homecoming match for the former Canadiens star goaltender Theodore and for defenceman Patrice Brisebois, who had two assists despite being booed.
The Avalanche (3-3-2) were coming off wins in Toronto and Ottawa this week.
It was as though the clock was turned back 15 years and the Avalanche were still the Nordiques as scorers named Stastny and Sakic led Colorado to a 3-1 first-period lead.
Sakic converted a Marek Svatos pass into an open side on a rush at 15:26 and set up Brunette's goal only 26 seconds later.
Stastny's shot on Milan Hejduk's rebound trickled past Aebischer at 16:42.
Souray got his first of the night with a point shot on a power-play that beat Theodore stick side at 17:20. Souray pounded in another with the man advantage 4:22 into the second frame.
Arnason, son of former Canadien Chuck Arnason, redirected a swiped shot by Wojtek Wolski into an open side at 6:26 but Samsonov got it back when he converted a Kovalev pass at 9:23.
Montreal was on its second two-man advantage of the game when Saku Koivu slipped a pass across the crease and Ryder shot into an open net to tie the game only 29 seconds into the third period.
Rivet had just come out of the penalty box when he was sent in on a breakaway to beat Theodore to the stick side for the go-ahead goal at 5:40.
Brunette got that back on a power-play at 8:43, but Higgins tipped in Koivu's rebound at 10:55, Kovalev got one at 12:48 and Johnson finished it off at 13:54 as a despondent Theodore absorbed more chants from the crowd.
Notes - D Kurt Sauer and F George Parros (shoulder) sat out for Colorado while Mathieu Dandenault (leg) and Aaron Downey (concussion) missed for Montreal. . . . John-Michael Liles wore No. 4 and Stastny No. 26 as Liles gave up the jersey so his teammate could wear his father's old number.