I handle this in a VERY formulaic way:
When you change playbooks, remove all of your old playbooks moves including the stat+1's. Keep the stat+1s from your list of standard advancements. Now you become the other playbook with the stats you've got left.
Then, you may elect to pre-select advancements from the standard list that you want to keep. If you want a move, mark off a move from another playbook. If you already had a gang, workshop or followers: keep a smaller one by marking the option off your list. If the list does not allow for that advancement, then you cannot bring the move with you.
Done.
I do this because I like that you have to pay for your stuff, and it makes advancing more of a lateral-story driven move, rather than some EPIC LEVEL OF LOOT AND POWAH thing. Also it contains the late game to a certain power expectancy. No Gunlugger can be a better hardholder because they classed in, then someone who was a hardholder from the get go.