Being physical, you would physically have to remove the previous vote first. This would require some combination of a chemical process, color matching, and/or glue piecing back on.

This all takes time, tight tolerances, and automation to a happen at an impactful scale, which minimal oversight would mostly prevent.

Compare this to an SQL statement to change a whole states worth of votes in a few hundred milliseconds.

You can always recount the paper.

Whatever solution, it require the limitation of involving something physical and difficult to manipulate.