First, I note that 2.1 is historic. However, 2.3 behaves in the same manner. See <http://www.openldap.org/faq/index.cgi?file=883> for a discussion of this error.
I note that, in 2.4, we hope to introduce a control which will allow administrators (and other authorized users) to request this (and other) restrictions be temporarily relaxed. In 2.3 (and earlier releases), the only way to change the structural class of an object is to re-create the object (e.g., delete the old, add the new).
--Quanah
-- Quanah Gibson-Mount Principal Software Developer ITS/Shared Application Services Stanford University GnuPG Public Key: http://www.stanford.edu/~quanah/pgp.html