Changing markings is no particular big deal. Winter instruments that have markings on the inner face are done with sticky back plastic tape that is easily removed and replaced. Wag Aero has the marking kit for just a few bucks.
Did one yesterday.
UH

Oh, yes it is a big deal. For a type certificated aircraft, glider or airplane, only a certificated instrument shop is allowed to open the instrument. Putting marking on the outside of the glass is OK but opening the instrument is not. Read the regualtions.

Robert Mudd
A&P I.A
Composite Aircraft Repair
Moriarty, Nm