Changing cassette

I have a 9 speed compact 34-50 with 12-25 cassette. Would like to make it a bit easier on the hills. If i change the cassette to 12-27 will anything else have to be changed? Also it looks as though Tiagra and 105 do not do 12-27 in 9 speed but Ultegra do, is this right?
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Don't know about Tiagra/105/Ultegra but you could also look at BBB from Parker International - could be much cheaper???
Ultegra will be a bit expensive, have a look at SRAM they do a 12-28 or something similar which will work fine also.
The only adjustment I had to make was to the b-tension screw as the jockey wheel was touching the largest sprockett when in the lowest gear.
worth it for me though, it makes the Derbyshire hills almost enjoyable !