Our MC says (as do the rules) that getting a new playbook means taking on a new life. You leave behind the old and take on the new. For a hardholder, you give up your hardhold. For a driver, you give up your car(s). For a gunlugger, you give up your guns.
Actually, that's not what the rules say; it gives examples of things that might change, but if you read the whole section on that advance, it ends with the following:
It’s up to you (the MC)
to work out with the player which of her character’s
things are in which category. (intrinsic vs old life)
What stays and what's left behind is decided on a case-by-case basis, unique for each character that takes this advance; it may be a radical change it may be subtle, but it's not cut-and-dried. Heck, I've had a player make a change in playbooks that allowed a
debility to be removed!
And, for your Gunlugger, they wouldn't be giving up their guns, they'd be giving up their
family if they got rid of their weapons; if the change to a new playbook doesn't involve the fiction of giving up that "family" there's no need to get rid of them.