if you never touch the house wiring, code is grandfathered.
when you make a change, by code, everything related to that circuit has to be brought to the current revision of code. if you have some old arcwelder panel like an FPE or God forbid, a Zinser/Sylvania panel that has multiple issues beyond being unable to accept an AFCI breaker, you are required to change out the entrance panel or otherwise provide AFCI to the whole circuit. and changing the panel makes every other circuit in the house subject to the requirements of code as it exists today. if that's something archaic like a 60 amp or a 120-volt single-phase power drop, that also has to upgrade.
only the local electrical inspector can waive any requirements in your area.
and this January 2014, every circuit in a living space is required to be on an AFCI protected circuit in the latest code book.
now, you can get there by making the first device in the circuit an AFCI device (outlet) and chain the rest off it like you can do with GFI outlets. assuming you can find the first device. which is why AFCI breakers are the go-to solution now.
not a licensed contractor, I got my understanding from home inspector blogs and contractor posts. your voltage may vary. any questions, the source is your local inspector, because communities often tweak the standard codes. if your burg has not adopted the 2014 NEC code, yet, or had it forced on them by state regulation, none of this matters right now.