Hey Mike
For now no data is ever deleted from Connect. There would be a big announcement if that were to ever change. At that time they would define the policy for things like how long data is held etc…
Projects (and the files they contain) simply become read-only if you exceed your project limit. I believe that your newest project remains “writeable” when you downgrade to a free account. I don’t think you have a choice which project remains writeable other than deleting newer projects (latest existing project is writable under free). You can’t move a project to a new owner nor can you migrate them to 3DW.
The biggest thing to point out is that it’s a good idea to plan ahead when you opt to end your subscription. Especially if you plan to continue under the free feature set. Just get your projects in order so you don’t have to copy things you still intend to make changes to.
Hope this helps!
Bryce