My experience (46 years of cycling and DYI on my own bikes) tells me that you should never put a new chain on a old cassette. It just grinds the s*** out of both. I usually replace the chain when the cassette is shot. The crankset gets replaced every other time, but if a guy was Rockefeller, you'd just change all the running gear out every couple years. I ride about 2000 miles a year.