Downgrading from a paid Workspace plan to a Starter plan will lock all un-hosted projects besides the 2 oldest projects (the actual first two sites created on the account).

It is not possible to switch the order of the projects to unlock another without adding a Site Plan to the project you would like to unlock.

At this time its not possible to choose which sites are locked when on a free workspace plan. It would be a nice feature to be able to choose which sites are locked.