As I have indicated in another nearby context, I don't believe that every organization with the word Institute in its name has to be seen as a continuation of an earlier organization.

*The* Institute of the Oikumene has a very particular ideology, which bears no resemblance to that of the Historical Institute of the Gaean Reach (at least, as long as it lasted; it only appears in Emphyrio [before the term Gaean Reach had been introduced] and the Durdane trilogy). I see no reason to believe these two organizations have anything in common.

The Fidelius Institute is, by name, a different organization.

The continuation of the IPCC is truly remarkable. In this case, I don't think Vance is giving serious weight to the span of 30,000 years. The human society(ies) of the Gaean Reach are all functioning about the same as in the Oikumene, there has been no dramatic technical or social evolution in the intervening interval. I believe that, after writing the first three DP novels, Vance adopted what I call an "ancient future" for the many practical benefits that offers -- lots of lost worlds and lots of bizarre cultures whose origins don't need to be explained in detail.

With any luck, I may be able to finish my essay on this topic and post it somewhere in time to mark the 90th year of the Vancean Era. I've always liked the German term Festschrift.