How do you know the "batteries need replacing"? Modern pacemakers don't just "die". When the battery voltage drops to a certain level. The pacemaker will go into an "end of life mode" (battery, not person). It's sort of a last ditch conservation mode until the battery can be replaced. What happens then is the pacemaker will fire to a preset pulse rate. It will no longer take into account what the user's heart is doing. The is reported by some to be a very uncomfortable, miserable feeling. Are you feeling this? Also, it is not the pacer batteries that are replaced, but rather the entire pacer itself. The leads are unplugged, from the old one, and the new one is "installed". The procedure is very similar to when the original one was installed, minus the lead placement.
I would say it is an emergency IF you are showing symptoms. IDK if the emergency room would do it unless you are showing symptoms but your best bet is to make an appointment with the dr that put it in.