There was a recent IAEI article per UL1699,on AFCI's. One section stated the listing was for new branch circuits only.
This surprised me as I am being told to AFCI all bed & living area outlets during a service upgrade here in VT. (Yes I know, the NEC is'nt retroactive..)
In asking UL, the listing is supposively being altered for 'old work'. And they are less effective on older circuitry at best.
It's nice to have rationale follow NEC changes, makes it easier to pedal as an end installer/marketer/justifier than simply saying 'because it's code...'.