Bottom line, you can use it. Carry proof of your employment under a certificate holder.
You are required to be employed by a Part 121, 125, or 135 carrier using an operating certificate granted by the FAA. You must also have completed the certificate holders approved training program for pilot in command qualification. This doesn't mean that you need to be in an upgraded status, or a captain; but you must have completed the PIC qualification program.
Many carriers type their SIC's initially, or at a set inerval after hire. If you undergo the same training that a PIC undergoes; if you have completed the company PIC qualification course or training and can provide proof of this, then you may use the old test.
That said, it's not a bad idea to keep studying the material, and to have a current test report available. Many firms use the ATP test questions verbatim as part of the prehiring screening; knowing that information and having it fresh in your mind is a good idea.
§ 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;...
...(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 as a flight crewmember in scheduled U.S. military air transport operations at the time of the practical test, and has accomplished the pilot in command aircraft qualification training program that is appropriate to the certificate and rating sought.
