Forget any nonsense you might hear about the cassette and chain needing to be changed at the same time, every time. It’s partly true, in that the chain and cassette wear in unison, and there’s a meshing that’s particular to a given chain and cassette. My experience ( depending on how many Joules you’ve put through the system, and the quality of the cassette and chain) is that you may get a bit of slip, with a new chain, on an old cassette, but it normally ‘re meshes’ after a few KiloJoules have gone through, and it’s not uncommon for a cassette to be perfectly serviceable, having had a couple of chains worth of use. Best practice is to change both cassette and chain together, but it’s not strictly necessary. I change cassettes and chains and Jockey wheels at the same time, but that’s just my personal preference.