If the chain is sized so that it can do big/big and small/small with an 11/28 cassette, then it will also work with an 11/25 cassette.

The big/big will have slightly more slack which won't be bad.
On small/small, is there adequate tension in the chain? Does the chain rub on itself?

If not, you'll be just fine with the old chain length.

Make sure your chain is good, or go ahead and install a new chain with a new cassette.

You may choose to adjust the B-Screw so the derailleur lands closer to the cassette for more precise shifting. Or, if it works, don't worry about it.