Assuming the synthetic airspeed is only a software update, that can be done on the aircraft quickly and easily - a few hours, including any functional checks/validation. Such software changes are fairly routinely done on an overnight.
Flight critical software updates are not real common, but are not exactly rare either - once every year or two being fairly typical. The main reason they are not more common is that the validation and certification of flight critical (i.e. DAL A) software is very time consuming and expensive. What tends to happen is less important changes/updates go on a sort of wish list. Then, when something important comes down (or sometimes if the wish list gets long enough) that justifies a change, they decide which of the wish list items should also be included.