One of the differences in attributes between Mark and Bryon is their sense of moral righteousness. Even in their penchant for mischief, there is a difference between them. Bryon's attributes served him as the "hustler," while Mark's served him as the "thief." This highlights the differences in terms of where they are on the moral spectrum. Mark is closer to being irredeemable, while Bryon demonstrates a propensity for maturation and reflection. Mark's attributes enable him to sell drugs, a line that Bryon cannot cross.
It is in this difference in characterization where their divergence takes place. Mark feels that there is no need to change and wishes to go back to the past, a time where he and Bryon were happy getting into trouble. The terse response that Bryon gives of "that was then, this is now" serves to highlight how he is changing. Bryon's propensity for change is what enables him to turn Mark in for selling drugs and send him to jail. Bryon's maturation and embrace of change is what causes him to feel so fundamentally alone at the end of the novel, a reminder that what he said was true in how "that was then, this is now." The traits of maturation and willingness to embrace what is as opposed to what is are where Mark and Bryon possess fundamentally different attributes.