I agree with NY that changing from a 12-25 to a 12-28 is probably doable without a derailleur change, but, to be honest, that's not much of a change. Personally, if you're going down this road, I'd go all the way to at least a 30 and a 32 or 34 is even better and Shimano does make a 10 speed cassette in those. Might have to change out your derailleur, but that's still a lot cheaper than buying a new bike. For sure, though, find a bike shop with a mechanic that knows the ropes on Shimano.