If they are still able to book appointments you might want to do the following:
You can duplicate all appointment types and make sure they are labeled private for now. Then once you're ready for them to be live you can make them public and add the links, embed, etc where you want them to be.
For the current appointments, you can turn them into a waiting list of sorts. Not sure how it's set up but in the description let them know that will be added to a waiting list and the date they put in lets you know whatever it is would like to know, "preferred dates/days" or "put on a first come first serve list". Could be a great way to collect information.
The other option is wherever these links are live for people to book. Create a form and make it a button to put people on a waiting till booking is available.