No matter the length of the stay in an indoor pen, the swans must be misted with warm water on a daily basis and kept out of drafts.
Once you know the target date of release back onto the pond, you will need to thoroughly soak the swans 2-3 days prior to their release to ensure that they can preen themselves dry. Otherwise, they could drown or get sick from not being properly oiled and water repellent. The Regal Swan « Hide