Chain length should be the same, but depending on the make/model of cassette, you might have to adjust the derailleur because the cassette will sit further in or further out from the centerline of the bike. If this is the case, you should be able to use a thin cassette spacer to space them the same. However, in all likelihood, it'll just work fine as is with no adjustment needed. Best way to find out is to try it. B-tension will not have to change, as the large cog isn't changing.

Most important adjustment is always lower gear (inner) limit stop. I always shift to the second-lowest (second from inside) gear, then slowly tug the rear inner wire by hand to make sure derailleur won't rub or hit spokes. But it sounds like you have that all dialed in. There's very little difference between an 11-25 and 12-25 as far as RD and chain adjustment is concerned.