Well, the original release did not specify the basis on which certificates would be converted to credits. Given that there are at least 9 different kinds of cerificates in effect at present, to suggest that *all* of them should be converted on a 1-to-1 basis would be pretty ridiculous. For example, the freebie certs that elite FF's get are good for 3000/5500km, which in the new system will require 2 upgrade credits.
In view of the wide difference between the old and new systems, I repeat what I said above: the only fair and sensible way to handle paid certs is to either re-credit the points or honour them at face value for as long as they are valid.