S2 Style System: Paid or Plus account users who are using a public style that is not available to Basic Accounts, or Paid users who are using a custom S2 style can continue to use that style, but will not be able to edit it further than the options on the Custom Options page of the Journal Display area. Paid journals will also revert from the customized comment pages back to the system default pages.
Why are only paid users mentioned in that second sentence? Plus users also have access to customized comment pages, and those will also be reverted.