The procesure is in the TSB that i linked above:
That bad luck. I don’t think it is very common i have not seen this happen here i think
This thread may help you. The member ended up disassembling is gauge cluster. You can see photos of the screen without the cover in his posts.
There are a lot of used speedometer on the market for a lot cheaper. You coud try to buy one disassemble it and swap the screen.
The mazda procedure to reset mileage for odometer replacement is here but i am not clear on what part the mileage is stored on.
In short if I understand correctly the speedometer needs to be programmed by a third party before the dealer receives and install it (SPECMO option). Otherwise it seems like the sticker is the only other legal options.
The document says the two options should have been discussed and given to you before the replacement, but it looked like that was a miss on their part.