What is the effect of changing the autobrake setting after touchdown?
This answer was difficult to find as both Flight Operation Manuals and Maintenance Manuals did not say. The only guidance was found in the system description provided by the OEM of the BCSU.  I am not mentioning the fleet type because at some point the detailed description might become proprietary.

According to the description I found for a given automatic braking system is that it is armed when the pilot presses one of the three pushbutton switches which set the auto-brake level.

The system will only arm if gear up is not commanded, there is hydraulic pressure as needed, and no existing faults exist that prohibit normal braking, as well as a few other computers available on the associated data bus. 

There is nothing that says the system can't be armed after touchdown, however the system will only engage when the spoilers are being activated, or are activated. Or more precisely from the black-box point of view automatic braking is engaged whenever two spoiler signals in three appear once automatic braking has been armed.  In other words, once armed, the automatic braking will only engage if spoilers are sending the signal that they are being activated, or are activated. 

It specifically says that the pilot can select another pushbutton switch even after
arming. The BSCU acknowledges this new command and executes
the corresponding task.

Automatic braking is released whenever the braking sequence
is interrupted:


when an arming condition disappears,
when an engaging condition disappears,
when one or both brake pedals are held down beyond the
defined threshold,
when the selected pushbutton is pressed again.


The question, therefore is 'After you have already armed at a given setting and the system is actually engaged, because the spoiler actuation has initiated auto-braking, what happens when you press another setting?'  As long as the arming conditions still exist, which in most cases would, and as long as the engage conditions exist, as would be the case if the spoilers are activated (at least 2 of three spoiler signals being sent to the BSCU), then yes, you may change the setting after touchdown and the BSCU acknowledges this new command and executes the corresponding task.