Yes! You can if you are security level 3 (Account Admin). To do so, find the user that you wish to update on the People tab. Once on that page, you should see a pencil icon on the left that will take you to the editing page for that user.  Simply change your user to the desired user type, and they will be changed. Click Save when done on the top right.

You can view a full detailed breakdown of permissions available by clicking the "?" icon next to "User Type" just above the dropdown to change between Helper/Planner.

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. (The free/included version of Service Planning through your ChMS allows you 5 Planner accounts). If a Planner is a master account or team leader, you will not be permitted to change him or her to be a Helper.