One of the ways work on old houses has progressed here in Minnesota, when adding new wiring in bedrooms, is to keep the old and the new seperate. Any new work in a bed must have AFCI from the breaker. . .we do that with new wiring including a new branch circuit. The old wiring that is still in the bedroom that is undisturbed by the construction is grandfathered in and left alone on its original overcurrent protection.
On service upgrades, only new wiring added in a bed is required to be AFCI protected.
It's a hassle to run the circuit all the way back to the panel, but it side steps this very problem that you are struggling with.
