Sure, you can change the status of a topic from Public to Private and vice-versa at any time. People watching a topic, be it private or public, do not get notified when you remove them from the topic or when the topic status is modified.
However, if you change a public topic into a private topic, only the people were watching the topic while it was public will have access to the newly private topic. The others who were in the public topic, but they were not watching it, will be left out of the topic. When they access the topic, Hubgets will let them know the topic status has changed and will recommend them to ask the topic author for permission.
Here's a screenshot that will help you change the status on an existing Topic: