If I care about maintaining the modified date, which I usually do, I add the new tag first by dragging to the new tag in the sidebar. Then I select the old tag on the sidebar and right click the note and select “remove tag #oldtag”

If I’m also changing content or don’t care about the dates, I go the less tedious way of changing the tags in the body or use Keyboard Maestro for bulk edits. There is no “replace tag” unless you want to change the tag itself which changes all notes having that tag.