I have several private trees on Ancestry.com and frequently copy a person from one tree to another. On the person's profile page, on the row of drop-down items under the person's photo, look for the "More Options" drop-down. Select "Save person to another tree". Then you can select the tree, chose an existing person in that tree, or save as a new person.
Go to the tree you just added the person to and go to the "Member Connect" tab. There should be a connection to the tree you just copied the individual from. It will let you add the media, sources, events (including descriptions), and family members.
It is a bit tedious, but if you tackle the project a few people each day, you should be able to move all the people to one tree without losing all the previous work.