Hi Nitin,
The copy form P to Q will not be be done in existing test tenant, but to a new one.
You can request for decommission of existing test (Q) system) and request a new one which can be copy of (P). As per my assumption, in this case you do not need to pay more.
But this conformation you can get it by creating an incident for SAP (of type Service Request).
You can also check below blogs for details on tenant strategy:
Cloud for Customer tenant strategies
SAP Cloud for Customer Tenant Landscapes: Determine how many tenants you need
Regards,
Suman