There are really only two ways to modify base schema in eDirectory. Used to be only one, which was to call Novell Technical Services (NTS) and have them use DSDump, (effectively a hex editor for eDirectory) and twiddle the bits by hand.
I have had them do this when merging two trees that had ALMOST but not perfectly compatible schema (Two flags on three attributes were different for no apparent reason, and were meaninglessly different).
The second possible way is to delete, then add the attribute in a single LDIF operation. That is a shown as a possibility in this TID: Changing schmea
The LDIF ought to look something like:
objectClasses: (220.127.116.11.5 NAME 'MyObjectClass' MAY (attr1) X-NDS_NOT_CONTAINER '1')
But in your case, replace the appropriate details with yours.
I suppose a third way, is when such a problem is identified and a specific fix is offered in DSrepair to fix it. (There was an old base schema setting for creatorsName and modifiersName that originally was DN syntax, but that was a terrible idea, as if you deleted the object that created users, the creatorsName was cleared, which caused all sorts of problems. There was a DSrepair addition that would recognize this error and fix it. If this turns out to be a real problem, they likely will offer this kind of a solution.
However until then, your best bet is to open an incident for NTS to dial in and use DSDump to fix it by hand. It literally can take a minute or three to do.