One roofer said to redo the flashing, by removing it and putting in new. The second said if the flashing is good, it should stay. They're both right because either practice is sound. If the flashing is bad or deteriorated or inadequate or causing leaks, then definitely replace it. If it is watertight and in good shape and there are no leaks, then leave it.
Roofers can put up new shingles with old flashing.
Replacing flashing requires removing all old shingles before putting in the flashing and new shingles, and some roofers feel that if this is being done anyway, why not replace the flashing. If a roofer has to go that far, he might as well take them all off. Q. I have some nice old tile in a bathroom with the usual missing tiles. The problem is finding them, which is why the handyman has devised his huge electronic file, listing a lot of them. Q. I am having a screened porch winterized with nine new windows.