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"Seniority" is a right defined in your Collective Bargaining Agreement. Some CBAs provide for a member retaining seniority when promoted outside of the bargaining unit, others do not.
The Federal Service Contract Act only provides that, if the contractor is operating under a successor contract (that means the contract with the federal government, not the CBA), then the wages must equal or exceed that provided in the predecessor contract. That provision is found in Section 4(c) of the Act (41 U.S.C. Sect. 351 et seq.)
Thus, if the predecessor contract purchased services that were covered by a CBA, and that CBA provided for retaining seniority rights after promotion to a supervisory position, then that right must be extended to the current employees of the contractor.
Otherwise, it is lawful to have a CBA that provides that seniority is forfeited when the member leaves the bargaining unit (as by being promoted to supervisor).
In short, you need to look to the earlier collective bargaining agreements to find out if seniority is protected by the Federal Service Contract Act.
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