Could I propose an alternate strategy for those who do not have easy and inexpensive access to a refill service:
If you are at all unsure of the viability of your old extinguisher, buy a new one and hang it right next to the old one. If you have a big fire*, use the new one first. If you have a little fire, use the old one first. If it puts out the fire, you still have the new one to use on the next fire. If you don't have a fire, in 50 years, you may run out of wall space for your extinguisher collection.
* If you have a fire, in the heat of the moment you will probably decide it is a big one. That's not an issue: You will now have two extinguishers that need to be refilled, which saves you overhead of transporting them one at a time to be serviced.