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sri_siri, Yes its unused approved LC which can be used for substitution, but the condition is that, the date on which the LC was filed for ex-employee, your educational Qual., skill set and experience should be the same or more on that date. There is some application required to be filled for that your Lawyer will complete that paper work, and its filed along with I-140, which means both the cases will work at the same time. Regarding the time frame I don\'t know.