You can change the group type (security or distribution) of existing groups with the Active Roles Console.
To convert a group to another group type
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In the Console tree, locate and select the folder that contains the group you want to modify.
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In the details pane, right-click the group you want to modify, then click Properties.
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On the General tab in the Properties dialog, under Group type, select the group type (Security or Distribution) you want to specify for the group.