Sometimes the flashing is chewed up badly and we are asked why not just pull the shingles and replace the flashing. That is the way a roofer would make the repair. The two problems you need to be aware of before using this approach is that the shingles may be old and brittle. If so the repair will likely be ugly, especially if new shingles that dont match are used. The second is rotted decking. If the flashing has been leaking for a long time the decking may be rotted, in which case there is nothing to nail a new flashing to. This is an expensive repair. Critter Quitters repair vents without the need to pull shingles, so none of these issues will raise their ugly faces.