Doc my 2 cents....
Don't mess with he speedo if it is functioning correctly.
There are many sensitive parts in there.
I messed with one of the ones I have and tried to roll back and could not figure it out. Seems easy but no thanks.
As for odometer reading. If you swap the head out for another the milage will be different, some states might have a form to fill out. But I doubt you will have any issues. Just tell them you replaced the speedo with another. DO NOT say you rolled it back.