
I do not know whether building codes address this but it's not an unheard of practice. If done well, it can lead to a well functioning roof. When flashings are replaced, I suggest leaving the old flashing if possible. Redundancy is not a bad thing.
Oftentimes that is not the easiest thing to accomplish on an existing home but, yes, it ideally should.
The contractor has installed new osb over the existing plywood sheathing. He installed the osb over the existing sidewall flashing. The new top layer of osb sheathing has ice and water that butts to the sidewall. Is this acceptable?
Does the new flashing need to go under the house wrap as well?