In order to support "Old DN format including DN in the double quotes" another cn=config switch may be necessary. It seems there was recently a new switch introduced to make the dn syntax validation a little more "relaxed" - nsslapd-dn-validate-strict. Maybe this one could be used to allow for DNs with double-quoted values?
Actually, the way how we are going to handle the old style 'dn: <type>="<nested dn>",<the rest>' is converting the old style to a new style in the normalization when the server receives DNs from clients and the converted new style DN is used in the rest of the process.  The nsslapd-dn-validate-strict value is examined in the DN syntax validation code for now.  Unless we change it, the DN syntax validation code always receives the new DN style.

Ok. What i wanted to say is that we should avoid any new config parameters in cn=config. The way you propose to handle the problem is the best one - it is completely transparent to the user, the server back-end "sees" only the normalised DNs so it does not complain and no additional configuration
parameters are necessary.

That being said, are you suggesting if nsslapd-dn-validate-strict is on, we should not convert an old style DN to a new style?  That'd be really strict.  I'm leaning toward to the other side accepting the both old and new style with no restriction.  Do you see any disadvantages in allowing the old style?
No, absolutely not, i agree completely with your reasoning. The code should be strict but not completely rigid :) Taking care of the "legacy" presentation in a transparent manner is the ideal solution.