I'm not aware of any controlling law or regulation that mandates the Program Office to provide continuing release of data for systems that are already fielded. But any capability upgrades or alterations that materially change system performance (and any that pose substantial risk of degrading fielded military capabilities if they fail) need to be tested by the Operational Test Agency (OTA). (DoDI 5000.89 ¶4.4.d.)

These modifications to fielded systems are development activities in their own right, so they need their own test planning and execution of further DT/OT. It is certainly in the interests of all parties to maintain continuous communication and share data that will support the continuing capability for the warfighters throughout the life of the fielded system.

You might consider documenting the working relationship between your OTA and Program Office in a memorandum. This could help ensure that the OTA is kept informed about planned modifications and current fielded capability while avoiding excessive or uncoordinated data demands on the Program Office.