Transmission wears into together, as such normally best to change cassette and chain at same time. If you have not done many miles though you can try replacing cassette only, recommend caution on first ride after fitting, gradually apply more pressure to make sure chain does not jump
Chain only goes around about half of the teeth on the back so if you have lost two overall (27 to 25) the chain is only going round one less tooth which is only one link of your chain.
As it has the capacity to allow for a difference of 15 teeth (12 to 27) it is adding in or taking out over seven links worth of chain between top and bottom.
I have been messing about with my bike in the other direction (23 up to a 28) and it has coped with that OK.
Depends how long the chain is at the start, and if you have a medium or short cage rear mech. It might be OK as it is, or you might need to remove a link. As for replacing the chain, if you change chains often then you don't need to replace the cassette every time - this would cost a fortune! However, if both chain and cassette were quite old then it's likely that you'll need to change both together. Replacing the chain often will prolong the life of the cassette.