It would be interesting to know if the manufacturers even plan to do the battery replacement of your 6-10 year old unit. That would require them to keep spares (that have a shelf life), and the labor to open, replace, reseal the units. You could imagine it might make sense for them to simply send you a lower end current unit.
-john
WRT replacement batteries, due to expiration dates, nobody really "stocks" these for any distress beacons. They generally build to order or once they have a period of consistent sales history, will build to anticipated sales based on that. It is a PITA for the manufacturers and PLBs are headed to becoming throw-away items (but, please don't actually throw them away in the trash without disconnecting battery). EPIRBs and ELTs are a different matter being used in mandated carriage and these they can plan on replacement with required inspections that ensure sales of replacement batteries for a period of time. In any case, no way this will ever be cheap done correctly and considering market size, etc.