The question stands, which service centers are willing to do that (certainly not Apple) and what are they going to charge for it? (I was charged about $300 for switching the HD to an SSD in my iMac, just for the work, the SSD came extra)
Any Apple authorized service center is “willing to do that”?
That is the question and their mode of business. I am not aware that they are required to perform work on random Macs brought to them and they are pretty scarce. Which brings us to point #2:
Also there is a big difference between “it can’t be done” and it “cost more than I’m willing to pay”.
Yes, it is better than "it can't be done". But it is a strong limitation of a computer, if the pure work price for exchanging a memory module or a SSD is $300. And requires to find a service center which can do the work and probably a longer stay there. Even on most laptops, such actions are a matter of a few minutes, and on most desktop even much faster and normally user-doable.
Why would an Apple Authorize service center not be willing to take your money? Has there ever been widespread reports of Mac service centers refusing to perform supported upgrades?
Unofficially, you can do it yourself.
https://everymac.com/systems/apple/imac-pro/imac-pro-faq/ima...