Be loyal to who your friend is trying to become, not just who they used to be.

Old friends hold a version of you that is years out of date, and they hold it affectionately, which makes it hard to shift. The jokes keep pointing at who you were at thirteen. For somebody actively trying to change something, that affection starts working as a weight, because the group keeps handing them the old self. Loyalty to the past can quietly become opposition to the present.

Notice what a friend is currently trying to be and back that instead. Drop the nickname, stop telling the story, ask how the new thing is going. You can keep the history without using it against them.