Yes you can, if you are are security level 3. To do so, click the "People" tab under the People dropdown. Once on that page, you should see a green box on the left that reads "Convert User Types". Simply change your user(s) to the desired user type, and they will be changed.
If you are using the maximum number of Planner accounts permitted by your subscription level, you will not be permitted to change a Helper to a Planner. If a Planner is a master account or team leader, you will not be permitted to change him or her to be a Helper.