Complete the process to migrate every Collab project in a Unity org.
Wait until you receive an email from Plastic SCM that the migration process has finished.
Re-open the same Collab project in Unity.
The Migration guide will pop up, guiding you through the process of converting your local instance of your Collab project into a local instance of your Plastic workspace.
Once it finishes, you can continue working on your project, using the Plastic SCM window to check in your changes.