Yes you can, if you are are security level 3.  To do so, find the person you'd like to convert on the appropriate People page (People >> Planners/Helpers), then hover on the "action/info" link in the upper right corner of the tab.  One of the menu options is "Convert...".  Click that menu option and follow the steps.

Converting a person doesn't remove them from teams or assignments.  When converting, you'll have the option to have WP notify the individual of their new login information, since as a new Helpers use an email address to log in and Planners use a user id.

If you are using the maximum number of Planner accounts permitted by your subscription level, you will not be permitted to convert Helpers to Planners.  If a Planner is a master account or team leader, you will not be permitted to convert him or her to be a Helper.