Couple newb questions. Got a new chain and the thing is skipping on multiple cogs. I replaced the chain with the same model - so I'm assuming that means time for new cassette? I probably should have replaced the chain some time ago - I've probably put around 2k miles on the old chain and cassette.

So I have a 12-27 and was just going to get same so to reduce hassle. If I got something pretty similar like a 12-25 would I need to adjust the DeRailleur?