When you define archive for any form, new form with type 'Archive' gets created. If any changes made to regular form gets synced to archive form as well. If you have set archive to None later, then that archive form changed to Regular form type and no more sync happens. So after that whatever changes you made to regular form doesn't get reflected on archive. In that case, you can not set same form as archive for the regular form unless its in sync. Ideally in case you want to stop archiving for some time, you should set to disabled without setting it to 'None'.
Thanks and Regards,
You can't export a backup (def file) of an Archive form. If you've made modifications to the Archive form you would need to create a View definition (not a Form definition). When you modify the primary form, it modifies the Archive form accordingly - which probably overwrites the modifications to the Archive form. That's when you import the View definition.