I'd say "yes", provided you complete the captain's upgrade at you Part 121 carrier.
Section 61.39: Prerequisites for practical tests.
(a) Except as provided in paragraphs (b) and (c) of this section, to be eligible for a practical test for a certificate or rating issued under this part, an applicant must:
(1) Pass the required knowledge test within the 24-calendar-month period preceding the month the applicant completes the practical test, if a knowledge test is required;
(2) Present the knowledge test report at the time of application for the practical test, if a knowledge test is required;
(3) Have satisfactorily accomplished the required training and obtained the aeronautical experience prescribed by this part for the certificate or rating sought;
(4) Hold at least a third-class medical certificate, if a medical certificate is required;
(5) Meet the prescribed age requirement of this part for the issuance of the certificate or rating sought;
(6) Have an endorsement, if required by this part, in the applicant's logbook or training record that has been signed by an authorized instructor who certifies that the applicant-
(i) Has received and logged training time within 2 calendar months preceding the month of application in preparation for the practical test;
(ii) Is prepared for the required practical test; and
(iii) Has demonstrated satisfactory knowledge of the subject areas in which the applicant was deficient on the airman knowledge test; and
(7) Have a completed and signed application form.
(b) Notwithstanding the provisions of paragraphs (a)(1) and (2) of this section, an applicant for an airline transport pilot certificate or an additional rating to an airline transport certificate may take the practical test for that certificate or rating with an expired knowledge test report, provided that the applicant:
(1) Is employed as a flight crewmember by a certificate holder under part 121, 125, or 135 of this chapter at the time of the practical test and has satisfactorily accomplished that operator's approved-
(i) Pilot in command aircraft qualification training program that is appropriate to the certificate and rating sought; and
(ii) Qualification training requirements appropriate to the certificate and rating sought; or
(2) Is employed by the U.S. Armed Forces as a flight crewmember in U.S. military air transport operations at the time of the practical test and has completed the pilot in command aircraft qualification training program that is appropriate to the pilot certificate and rating sought.
(c) A person is not required to comply with the provisions of paragraph (a)(6) of this section if that person:
(1) Holds a foreign pilot license issued by a contracting State to the Convention on International Civil Aviation that authorizes at least the privileges of the pilot certificate sought;
(2) Is only applying for a type rating; or
(3) Is applying for an airline transport pilot certificate or an additional rating to an airline transport pilot certificate in an aircraft that does not require an aircraft type rating practical test.
(d) If all increments of the practical test for a certificate or rating are not completed on the same date, then all the remaining increments of the test must be completed within 2 calendar months after the month the applicant began the test.
(e) If all increments of the practical test for a certificate or rating are not completed within 2 calendar months after the month the applicant began the test, the applicant must retake the entire practical test.
