From FCOM VOL II B737-800:
"Landing autobrake setting may be selected after touchdown prior to decellerating through 60kts of groundspeed."
And further down:
"To maintain the selected landing deceleration rate, autobrake pressure is reduced as other controls, such as thrust reversers and spoilers, contribute to total deceleration. [B] The deceleration level can be changed (without disarming the system) by rotating the selector. [B] The autobrake system brings the airplane to a complete stop unless the braking is terminated by the pilot."
In this contex, does it suggest that you can change it after touchdown? I think it does.
Don't know about the Classics.
Greetings, Ziggy