Hi, All the evidence I could find seemed to suggest they are indeed synonyms. The difference between ModDN/ModRDN really comes down to the presence (or otherwise) of the "newsuperior" attribute rather then the value chosen for the "changetype" attribute.
I found this http://www.novell.com/documentation/ndsedir86/index.html?page=/documentation/ndsedir86/taoenu/data/a6i0f1p.html section "Table 143. Modify DN Change Type Fields" to be a good summary of related attributes (but don't have time to cross reference this with relevant RFCs etc).
