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.

Not really. Best practices in the field involve having representatives from many to all parties present at each step of the voting process, from the distribution of ballots at the booth, to the counting, to the archiving of counted boxes pending recounts.

In Washington State, photographs of each ballot are taken as the ballot is removed from the envelope and after it has been electronically tabulated. This prevents someone from replacing a paper ballot along the way.

Good thing photos can't be deleted. Oh wait.

Photos can be deleted. It would be detected and then law enforcement would or would not do anything.

A blockchain voting system would enable us to detect vote tampering, and then law enforcement would or would not do something about it.

What's the difference?

IMO, 99% of the people wont understand a blockchain system. To them it will just be "X won because a computer said so". So the question is do we want society to be filled with people thinking "I know those votes were made with pen on paper and were counted by my neighbors and trusted friends" or do we want society filled with people who think "it must be true because a computer said so"?

Not across thousands of voting places.