A member type's category (individual or organization) cannot be changed after creation. To switch, create a new member type. See Member types overview and default settings for details.
Note: Group attribute security settings cannot be copied across member type categories. For example, if you recreate an individual member type as an organization type, the Copy a Group's Permissions dropdown will not carry over the original permissions. Reassign them manually in Database > Attributes > Group Attribute Security.
To add members to your new member type in bulk, use Profile Import.