In general, FDA does not "decide" to change the dosage of a regulated medication of vaccine. In general, the manufacturer submits a desire to change, and, as appropriate, data to back up their claims of safety and/or efficacy with their request.
Here is a link to an article from way back in January on this topic:
https://www.cnn.com/2021/01/04/health/fda-coronavirus-vaccines-doses/index.html
Likely there are others.
EUA is unusual. This global pandemic is unusual.
One might ask: "Why would a company like Pfizer submit an application for the vaccine dose labeling to change to single versus double-- wouldn't that mean they'd make half as much money?"
And a related natural question might be "If Pfizer wouldn't seek a dosing change to their labeling, and public health experts and/or officials at CDC and/or NIH or even FDA did not agree, could one or more of these government entities basically force Pfizer to change their labeling?"
And then if such were possible "would the gubmint have to then indemnify Pfizer in some way?"
All of this might by immaterial if we did not have capitalism and our current legal system.
These are complex topics. In many ways, far too complex for easily distillation for public consumption.
I have a lot of experience at the intersection of industry and FDA regulation (industry side).
I remember back months ago when folks in these Forums thought that it should be easy for essentially laypersons to pitch in and come up with ventilators. Great motivations. Ignorance of details.
I remember back in '17 when by chance I met the brother of the man that was running ODE at FDA at a business luncheon in Sarasota . That man, an engineering consultant, had been a college roommate with a gentleman that was heavily involved with IMSA on the tech side (and later was in charge of tech). In turn, that gentleman was and still is a close friend of Jerry Woods.
By the way, FDA ODE is Office of Device Evaluation. Gatekeepers.