HI Sudeep,

Please clarify what you are actually doing as you say you are trying to change the Customer ID (or customer) on an existing contract but then are updating the address which does NOT change the Customer ID, only the addresses associated to the contract.

Please note, standard does not allow you to change Customer ID on a contract but you can change the shipping / billing address linked to the contract but this is not the customer.

I suspect you have corrupted data involving the creation of a new address ID which is what happens when you do this action.  The address_xref table has references to the BillTo or ShipTo addresses to the contract which the columns table_key, table_name, address_type_id are used to provide a reference to the actual address row. Perhaps the formula_value.last_value is not correct?

I suggest you log a ticket to have support review this potential corruption but again I don't see you actually being able to change who is the customer associated to the contract as this is not standard from what I see below where the option is disabled.

I was able to change the shipping address without any issue on this contract which is why I think  you have corrupted data.