Your contract with your client is for a current owner search for a fee unpon which you have both agreed. An implied term of every contract (At least in Connecticut) is that the contract will be performed with reasonable care. If the industry standard for reasonable care requires the extension of time for the search from a current owner search to a full search then contractually you would have to perform a full search.

However, you should notify your client that a full search is required, and a larger fee is required before performing the search. In effect it is a contract modification. Failure to do so would allow the client to insist upon paying the current owner search fee. Get the search order amended to show the increased fee.