You want to be careful steaming on old plaster. That will pull in any moisture and hold it. Then anything you do over the top will be fucked and have to be done again. Ask me how I know.....
We had two rooms replastered. The first crowd steamed the paper off the walls and skimmed over. That plaster all blew due to the excess moisture now in the walls. Had to all be hacked back to brick and redone.
You want to be careful steaming on old plaster. That will pull in any moisture and hold it. Then anything you do over the top will be fucked and have to be done again. Ask me how I know.....