crusintx, Your friend would likely have the tools needed to remove either the crank arm or the cassette. Find out from him what other tools you'll need for the whole fleet.

The difference between the 12-25 and 12-27 9S cassettes are the lower two gears, 23T to 24T and 25T to 27T. That keeps the most commonly used middle gears the same. If you're going to replace the inner chainring I'd suggest the 26T with your present cassette. You might need to shorten the chain. Because there's so much redundancy between the two cassettes an inner chainring swap makes sense to me.

Brad