I replaced my chain last week because a link in the old one snapped. When I removed and measured it, it was stretched by 1/8". There was no noticeable wear on the rear cassette so I left it with the new chain. The first couple of commutes this week were fine., Towards the end of the week, I noticed the chain slipping / delayed change in the most common gears. Have done 90 miles to date on the new chain. Is it too late to change the cassette - i.e. should I let them both wear out together, or should I replace it now?