Well, in my estimation, some drivers don't get ruined. If a technician is dextrous enough, the acrylic can be shaved down to manageable widths around each of the drivers, allowing for limited reassembly inside a new shell. It might not work for every ear, especially if the new ear is really small, but I think it's possible. For the same user to reshell, I think it's definitely possible. FitEar shells aren't completely filled; I've seen a Private 333 with an accidental crack in the shell, and the interior is hollow. It's just that the FitEar M.O. is to place drivers close to the front of the canal (most likely due to compensation for tube and canal resonance due to the shallower fit design), that and the triple-bore design requires a very thinly cured shell, so they have to add in more acrylic later. However, contrary to popular opinion, the shell is not completely filled --- that is a myth (that I erroneously helped popularize back in the day). If people remember user zowki's unfortunate accident a while back, they they'll be able to see that much of the driver assembly is salvageable, at least for a TO GO! 334.
I believe most, if not all FitEar's universals are only half filled with acrylic, which is the case of the Parterre and TG334 (haven't touched the F111 but I reckon there would be no difference), contrary to what we often see with their customs. And if my guess is correct, back in the day FitEar didn't completely fill their customs with resin. I've seen several 333 and even some MH334 in hollow shells with no internal silver wires but there was probably a point in time when they switched the production process and we ended up with what we have today. The chance of reshelling a recently made customs (completely filled with acrylic) is very slim, the problem isn't about removing all the resin covering around the metal casing but it's about how to keep the soldering pads of the drivers intact. That's the weakest part of a BA and most drivers are ruined when they're scavenged improperly due to broken pads.
The last possibility is that some of the drivers that Suyama has ordered are getting 'leaked' into the grey market. I've seen drivers specific for JHA and Westone being sold on Taobao, If people recall, the UM Miracle used surplus parts from the JH13 at the beginning. Of course, UM had to scratch off the markings because those drivers weren't meant for them. The Heir Audio 6.A was a limited production run for the same reason. Knowles accidentally sold a proprietary DTEC driver (not sure from which company) to Dr. Moulton. Thus, it's not entirely out of the realm of possibility that UM/Rooth has the ability to get its hands on drivers meant for FitEar.
Of course, all this is speculation. I have no substantiated evidence that would support the definite reality of any of these possibilities.
I'm pretty sure UM scrapes off all of their drivers, except for dynamic ones, regardless whether they're proprietary or not.