The below should work for what you're wanting.
Global
Triggers: onCPayUserStateChange
Type: CB Paid Subscriptions
User: Automatic
Access: Everybody
Conditions
1: [var3] Equal To PAID_PLAN_ID_HERE
2: [var2] Equal To X
Action
Mode: Subscribe
Plans: SELECT_FREE_PLAN_HERE
That should subscribe them to the free plan when their paid plan expires.