In That Was Then, This Is Now, what does Mark mean when he says, “That was then, this is now”?

At the end of the novel, Bryon goes to visit Mark, who is in a reformatory. While Bryon is sitting across from Mark in a visiting room, Bryon says Mark looks hard and sinister. Bryon asks Mark how his life is going, and Mark replies by commenting on how terrible it is in the reformatory. Mark then tells Bryon he wanted to see him just to make sure he still hated him. Mark doesn't allow Bryon to explain himself and says he can't get away with things the way he used to, which is why he's constantly in trouble. Bryon becomes desperate to connect with Mark and says, "We were like brothers. . . You were my best friend" (81). Mark responds by laughing at Bryon and says, "Like a friend once told me, 'That was then, this is now'" (82). Mark's comment refers to how his relationship with Bryon has permanently changed. Bryon and Mark used to be best friends, but after Bryon called the authorities on Mark, Mark's attitude towards Bryon changed forever. Mark no longer views Bryon as a friend and the fun times they used to spend together are in the past.