It will probably be OK.
Check to see that you can actually change into 50/28 as if you can't, and you accidentally select it out on the road, you could break something. If you can't select big/big, you'll have to add another link to the chain.
The other problem is that worn chains slip on new cassettes, so if you've got a 11-28 that you just use occasionally, you could end up using it at a stage when the 12-25 and chain are worn too much. Either change chains sufficiently often that it doesn't get that worn, or keep a chain for each cassette, and swap the chain at the same time as the cassette. This is easy enough with powerlinks.