We don't know if Harbaugh is willing to keep Denard if he takes the head coaching job and whether Denard will accept that. The best sell would be to convince Denard that he could be Mike Vick, with him passing more by running around the pocket. Another angle would be to tell Denard that this approach would reduce the possibility of injury. That way, Harbaugh keeps a little of what he prefers and Denard sticks around. Then, Harbaugh can make a cleaner transition (though still a spread) with Gardner.
We also don't know what kind of defense Harbaugh's going to bring in and what specific steps he will take to significantly improve the talent on defense. Again, all of these unanswered questions need to be answered BEFORE any change is even contemplated. I would even have no problem asking Harbaugh to talk to Denard and other key pieces before making the move.