I'm looking to upgrade a used MTB bike I'm considering purchasing, it's a 7 speed Deore and I want to change it to an 8 or 9 speed. Would the only change I need to make is change the twist shifter from a 7 speed to an 8 speed or 9 speed with a change of cassette? If I decide to change to a 9 speed, will I also need to change the chain?