Exactly how much wear your chain has will dictate your available options. On terribly worn drivetrains, the teeth of the cogs will begin to look hooked from the chain wearing high on them. If you’re at this point, replacing just the chain simply isn't an option.
Some riders will choose to just let everything wear out until their bike starts skipping gears, but your risk of a snapped chain increases with this. Others will replace chain and cassette, and hope the chainrings are in a re-usable condition. If you do this, any shifting issues, chain suck (where the chain doesn't disengage from the bottom of the chainring) or chain dropping will signal that the chainrings aren’t happy.
“If you're running a basic stamped steel cassette you could probably go past one percent wear and be fine," says Quade. "[But] if you're using an aluminium weight-weenie cassette it could be toast at a 0.5 measurement – the important thing to do is to test your bike after a chain swap.
"When the cassette is worn to the point it won’t mesh with a new chain, it can create a safety issue. So a [quick] spin after you've swapped the chain is never a bad idea. Never allow a new chain to wear into a worn cassette.”