Seems like draining, examining and dumping less than three liters of gear oil is a small price to pay for peace of mind, and a likely best practice when settling in a rebuilt gearbox.
When I had Bridget's transaxle replaced with one rebuilt to my specs for the Subaru engine I think the old one had the same oil in it since the Carter Administration. It was fine. The new one was fine. I changed the oil after the summer season and what came out was fine, so I put in new and drove on.
The Spyder box, having sat dry for a decade after its rebuild, then gotten re-worked again three years ago, will get the same treatment. I mean, why not?