The AirPods Pro received a firmware update shortly after the release on December 16 but a number of users complained about worse noise cancellation. Turns out, there is a good reason as to why Apple pulled the update. So not everyone who uses Apple's latest wireless headphones receives this update.
More specifically, the AirPods Pro show a significant drop in isolation in the bass-range, meaning they won't be able to block the sound from the engine of an airplane as well as before, for example. If you have already installed the update on your AirPods, there is not much you can do except wait for Apple to roll out another update.
The update did improve the bass accuracy of the AirPods Pro according to the site. This means that with ANC turned on, these headphones won't do almost as good a job blocking out the low engine rumbles of planes or buses as they did before this update.
Did you install the 2C54 firmware on your AirPods Pro? Other changes noticed after the firmware update include a "very slight reduction in mid accuracy" and "a very slight reduction in treble accuracy".
There is no indication at this stage as to whether Apple plans to re-release a fixed version of the firmware, or when it might do this. So it may be working on the issue described by RTINGS. You can find out which firmware your headphones are running by heading to the "About" section in your iOS General settings. But in fact, users can not upgrade or downgrade the headset.