2019 Mac Pro SSD

First off, the 2019 Mac Pro SSD is not really an SSD per se like an M.2 SSD. A single blade module (256GB version) or paired blade modules (1, 2, 4, and 8TB versions) and formed into an SSD via the controller SW inside the T2 security chip. Apple paired blade modules, e.g. > 256 GB size are label #1 and #2 and must be placed in the corresponding slots inside the Mac Pro. T2 software reinstallation requires a second Mac running Apple Configurator 2 and a USB-C cable compatible with Mac Pro (2019). The following support article defines the replacement procedure, https://support.apple.com/en-us/HT210626.

Hope this helps..

Currently, the only source for these upgrade modules is directly from Apple, and they demand you return an old set in exchange.

So if you have several, you can play "musical chairs" with them, but at the end of all swapping, you MUST return an old one to Apple.

Not necessarily true. This search criteria "Apple SSD Kit for Mac Pro (xTB)" where "x" is 2, 4 or 8 on Amazon will expose 2, 4, and 8TB kit results in stock and available as Prime shipment. I've also seen the 1TB kit available.

"I found nothing on the Apple site about that and I'm not sure how they could even enforce it."

Exactly. It's not like Apple rents the SSD module(s) when a Mac Pro is purchased. Apple could entice the returns by allowing a percent discount to be applied when replaced modules are returned otherwise they are the user possession to do with whatever they want. Or they could add an up front surcharge to the purchase to be refunded at time of return.

Previous policy for almost all Apple parts was exchange, as I stated.

It is possible Apple has relented and changed the way they deal with these particular parts.

I stand corrected.