Apple has just quietly announced and provided a way to increase your system SSD storage after you bought your 2019 Mac Pro. So you don't have to max out your system storage when you buy the Mac Pro if you're not sure how much you might need right away.
It is do it yourself, and you don't have to bring it in for service. It does require using a second Mac running Mac Configurator 2 (free).
Note that this is not for adding additional storage in the other slots, which can already be done in various ways. This is specifically to replace the pair of Apple proprietary system SSD's with larger capacity after the fact, such as going from 256 to 1TB, or 1TB to 4TB, etc. I haven't seen the details, but you order the drives from the Apple Store. Note that in each case it is an extra $200 more than if you upgraded at the time you bought the Mac Pro, which isn't too bad I guess.
I assume the Configurator software handles whatever needs to be done configuring the T2 security chip and all of that. I assume you would also need to back up whatever you need to and so on.
Here is the link to the option on the Apple Store:
https://www.apple.com/shop/product/M...B4b82%252B464d
And a brief announcement about it on 9to5mac:
https://9to5mac.com/2020/06/15/macbo...u-ssd-upgrade/
Note they have also added another graphics card option for the MacBook Pro.