You may change the Document Checklist Type for a plan using the Convert Plan function in the Plan Menu in the top right. You can change between different retirement checklist types, or between welfare checklist types, but if you need to convert a plan checklist from a retirement plan to a welfare plan, or the other way around, you must first convert the plan to a 'Misc' type, then complete the conversion.
Designated Admin users may Revert converting a plan from the Show History menu as well, but you cannot revert changes made to the plan prior to conversion without converting the plan checklist type back, since the Revert function rolls back the Plan Checklist to a prior state.