well- when my cartridge of Durable- that I had had less than a month and not even run half the resin in stopped working. Formlabs sent me a replacement cartridge full of resin, free.
that’s the one I cycled the remaining resin thru.
When the resin is old- and can’t be mixed with new resin… i think that may be as much to ensure you don’t ruin a lot of prints or the tank as much as trying to sell more resin.