Not sure WHY you want to update the records.
Having the bath added without permit depends on your area - in some areas they get nasty, in others only if they catch you actually doing it.
You could check on prior building permits for the house - many agencies keep them almost forever by legal plat number, so that could show the history - you could just tell them you bought the house and are looking for any old plans before you get into a remodel.
Adding the bathroom officially will increase your property tax bill.
Personally, I would not stir the waters since you did not do this - I would get your plans worked up by your architect, apply for the building permit, and if they figure out there is currently an extra bathroom then see where that leads. You could talk to your architect about this - expecially if you are looking at remodeling the added bathroom, maybe including it in the addition/remodel plans would be a solution.
