I assume you meant you were going from a 12/25. If the chain was sized as small as possible with the 25, then you need to add 3 links. It's likely there's enough excess chain totake the 28.

Put the 11-28 on the bike. Put it in the repair stand. Gently put the bike in the 53/28 combination. If there's enough chain to go into that combination comfortably, then your fine. If it's a stretch to get it in the 53/28, you need to add chain.

Even if you never cross chain into the big/big combination when actually riding, the chain needs to be long enough to accomodate that combination, under load, or bad things will eventually happen.

You may need a new chain anyway. If you do, size it to fit the 28, and you'll be fine running either the 25 or the 28.