The 'SR-72' timeline makes for interesting reading, LM was talking about it for a while when it was their private concept then suddenly scrubbed all mention of it from their website a few years ago, which some suggest that it became an official government funded black project. https://en.wikipedia.org/wiki/Lockheed_Martin_SR-72

Not sure why they would need an old SR-71 unless maybe they plan to test out something like new generation engines at very high speed, maybe with plausible cover to hide the newer more secret aircraft as flying at Mach 3+ tends to cover a lot of distance so it would be difficult to keep within secret test areas.