What GM has said is that the bricks can be mixed and matched. They did not say the consumer (you in your garage) would be doing this. These battery packs are not like removing two AA from your remote control. They must be disconnected from the car and dropped (they are very heavy) carefully and opened up. They contain lethal voltage and must be serviced with that in mind. Chevy technicians have special equipment, training, and special gloves for all this.
Swapping in a new brick would be a dealer doing this during the warranty period, not you. After the warranty, well, voiding a warranty by doing your own battery refurbishment is a moot point. If you or whomever you hire has the tools, knowledge and expertise, go for it. By that time there will also likely be lots of used bricks in the secondary auto-salvage market you could use instead of a new brick via an online parts seller, assuming they would sell by the brick.
The main point of the Ultium is NOT that you the consumer will be personally swapping in new bricks whenever the mood strikes you. No, the point is to be able to refurbish the battery even if the bricks are no longer being produced. Instead, the dealer could swap in the current Ultium bricks even if they use a different chemistry. This addresses the issue Volt owner's (and other EV's) are having as their 10+ year old battery's need some modules replaced. The modules are no longer being produced, and you can't swap in Gen 2 Volt modules or Bolt modules in the Volt Gen 1 packs. The Bolt will face the same issue at some point: lack or fresh battery modules.
With Ultium, as long as the bricks being produced at the time meet the physical size/archecture requirements, they can be a suitable replacement. The bricks could even potentially be solid state.