Thank you for your answer !
I’ve been spending some time on the subject and I think there is no way to do this quickly, unfortunately.
As you guessed, changing the name of the DLL didn’t work either. I may be wrong, but it seems like those unique IDs are not only related to the plugins, but to the presets themselves. Two identital track presets, with an empty instance of Kontakt 5 for exemple, will have different IDs.
Looks like it’s a dead end ! 
It’s not too bad in my case, I wanted to “update” my orchestral template using only Kontakt 6, but I will finally keep all my existing tracks with Kontakt 5, and use Kontakt 6 for the new ones.