Join Date: Oct 2007
Location: West Yorkshire, United Kingdom
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Unless you have an 'GS' model RD, then you will need a new RD, virtually guaranteed you will need a new chain as well, as the current one will be too short (you can confirm this by measuring the old one with the new cassette fitted) if the current chain has a lot of distance on it / wear would replace anyway.
On the basis you are getting a Shimano 4500 11-30 cassette, there will be a big jump from the 26t to 30t, if you are climbing unloaded (i.e. just you and the bike) you may find you have lost some useful mid range gears and gained one very low one.