If you downgrade to a plan that supports fewer social channels than you currently have connected, any channels above the limits of your new plan will become locked. We would recommend removing any channels that you no longer need, which will then unlock the channels you're looking to keep.
If you'd rather not delete any channels (for example, if you would like to keep them connected in case you upgrade again in the future), you can choose which channels to unlock using drag and drop. Drag the channels you'd like to unlock to the top of the list on the left hand side of your dashboard. The social channels nearest the top will unlock and the channels nearest the bottom will lock.