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.
The SR-71 is mostly analogue. It has remarkable battlefield resilience including ability to operate when EMP and nuclear radiation are issues that would affect a lot of digital systems.
Worried about how to navigate if the GPS systems gets knocked out by an adversary? SR-71 has an analogue system, use that. Worried about how an adversary could use cyberwarfar against your avionics? SR-71 has you covered, use all of that.
And sure, you could take all the designs and hand them over to the SR-72 team at LM and say "yeah, all of that please", and they could figure it out, but you know what might be handy? A working SR-71 they could play with. And who knows, maybe the answer is to take an SR-71 in its entirety, and just upgrade a few parts to get the SR-72 performance (Mach 6) that you're after.
Side note: IIRC a lot of the star tracking tech still exists and is used by Trident D-5, B-52, B-1, and B-2 etc. IIRC. Most of it is analog, although I think only the B-52s version is a mechanical computer assuming its still in service. You never design your second strike capability to require a system that will be knocked out in the first round.
Crucially, the engine inlet spikes were computer controlled. And since it's an observation platform and not a bomber, is it going to be EMP hardened? That was probably classified at the time. /shrug
The SR-71 mission set included trans-attack reconnaissance for SAC, where it would overfly a radiating Europe and USSR perform nuclear bomb damage assessment for follow-on strikes. Aside from nuclear bursts and leftover detritus from the ICBMs etc., it was anticipated to operate in an environment where nuclear-tipped SA-5 and other Soviet SAMs would be bursting here and there. It's EMP-hardened.
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.