I agree that we don't know how the tiara was meant to be converted. After all no one has published about that aspect. Perhaps, as you suggest Nellie, it can no longer be converted. I certainly don't know.
What I was trying to address was how the tiara might have been converted based on the information in Garrard's ledger, a copy of which you posted in the link I gave above. I did not base my opinion on the current shape of the tiara, although I cannot see that it has changed, except for being shortened and having the opals replaced. I should have stated clearly in my earlier post that my interpretation was not meant to suggest I was putting forward a definitive solution/answer.
In the following Royal Collection site http://www.royalcollection.org.uk/eGallery/object.asp?category=284&pagesize=60&object=200174&row=373&detail=about if the image of the tiara is magnified, it is possible to see some of the metal work from the inside of the tiara, and it is obvious that the arches, as well as the upright elements, are joined to the base by what looks like one mechanism. But perhaps I am just imaging things. No doubt others will have a keener eye than I have.