You can change the chain when Park Tools tell you to and buy 3 chains for a the life of a cassette.
Or... leave the chain going, and buy 1 chain for the life of the cassette.
From my experiments, the cassette lasts the same length of time. Despite arguments that changing the chain frequently extends the life of the cassette. I'm convinced now this is BS. What it does is simple allow you to run a new chain on an old cassette if you want to change the chain, without it skipping. Once you go beyond the .75 or even 1.0 wear level then a new chain will skip. But keep going with the old and it will still go and I bet you the cassette will last just as long before it all starts falling apart.
Thing is, yes, if you want to put a new chain on then yes you'll need to change cassette also unless you change chains frequently. However those chains are perfectly fine. If you let them run, they'll mesh with the cassette fine, both worn. Just means you can't stick a new chain on, but if you have a decent chain (like KMC) it won't self destruct and you have easy links to repair any that break. So stop paying money to Park Tools for their checker, who I'm sure are getting some kind of back hander from chain manufacturers
Oh, and I wonder why Park Tools changed their checker from 0.75 / 1.0 to 0.5 and 0.75 ?