I've had my Eagle XX1 gruppo for a year now and thought I could get away with fitting a new chain (Park chain-checker showed under 50% wear) but it skips under hard load in gear 4. Obviously I want to prolong cassette life as much as possible and can see 2 possible options - continue with the previous chain, keeping it clean/lubed or fit a chain that is partially worn (the latter is a longshot which I've never tried!).
I had this recently with my GX eagle setup. It's a shame Sram don't sell parts of a cassette as with 1x setups you spend so much time on the small end of the cassette and they wear a lot faster.
I'm back on the old chain until it dies, then I'll refit the new chain with a new cassette.
I'm curious about this. I nearly changed the chain the other day because it must have 1000kms on it by now, but I measured it with two different chain checkers and it's showing nowhere near worn (can onlyget the Park CC-2 in it without a little pressure even when it's fully "open"). I guess your experience suggests I should keep on truckin' with the old one.
I changed my chain, despite what the chain wear checker said. It's been terrible since. I think I might have to swap the cassette or put the old chain back on 🙁
The best approach is to use at least two chains in rotation, with the first change around 600kms or so. Chain checkers are a waste of time; if it skips and you don't have an appropriately worn chain that doesn't skip, it's time for a new cassette.
Also, keep an eye on the chainring wear as well; sometimes it can be the reason why a new chain feels notchy.