Best way to do that is to use parent > child plan setup. Example as follows.

Plan 1 (exclusive, free, lifetime)
- Plan 2 (non-exclusive, paid)

They'll always have Plan 1 in this case even if Plan 2 expires. Another options is to use CB Auto Actions to give them a subscription to whatever plan you like when their paid plan expires. To do that you'd use the following in CB Auto Actions.

Plan Expired
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
Condition 1
Field: Custom > Value
Custom Value: [var3]
Operator: Equal To
Value: PLAN_ID_HERE
Condition 2
Field: Custom > Value
Custom Value: [var2]
Operator: Not Equal To
Value: A

Replace PLAN_ID_HERE with the plan id of the plan you want to trigger this for. Next set Action to CB Paid Subscription. Under Action you can then configure it to subscribe them to whatever plan you like. This will trigger when PLAN_ID_HERE expires.

You wouldn't use CB Auto Actions for that, but instead would use the built in conditions features for plans. Edit your plan within CBSubs > Plans then under the Workflows tab you should set a "Registration CB workflows" fieldset and within that is a "Dynamic Conditions on other plans and CB Fields to show this plan and to allow subscribing to it" fieldset. That is where you'll be able to configure dynamic conditions for plans display during registration. Example as follows.

Conditions: These conditions must be met on the registration form to show this plan at registration:
And following CB Field: Subs_Plan
Condition: = (Is equal to)
Value: Category A

This should only show the plan if the field Subs_Plan has a value of Category A.