Following my previous post : I took the plunge and purchased the C12 update. This, to keep a functional Cubase version VST 2.4 compatible and allowing its use without the eLicenser USB key to work with it. I 'll describe the process, as maybe it could help anyone who installed the C12 Trial version without any clue about how to purchase the ‘regular’ one, as there is not (AFAICS) a single document about how to proceed…
So, not seeing any way to update, I went in the Steinberg site and directly manage to purchase the update (after having to go into the process to retrieve my password) and used this one (with trembling hands - I really wasn’t at ease at this point, wondering about my C11 licence, and Echanger Download Access Code means absolutly nothing in decent french. So, I just guessed that it was the way to go to activate my licence purchase) :
This done, and after entering the DAC received by e-mail, I immediatly unactivate, first, the Trial version (“version d’essai”) and, second, activated the other one and got the following screenshot, in SAM (Steinberg Activation manager) :
So, I guess that everything is good, but I’ll be sure of this only after next 30th september, as nothing in C12 allows me to know if I’m still using the trial version or the purchased one. Just hope that I won’t have to reinstall everything next 1st october…
I can’t resist to make an added comment, at this point : hovering the mouse near the red button in the SAM window, I stumbled on this very well hidden option (the cart with the blue background) which would have probably allowed me to purchase the C12 update directly from the trial version, instead of scratching my head about how to proceed. This is typically the kind of thing, that complicates things uselessly. No wonder that there are so many inquiries/threads in the forums concerning licences management, with this kind of implementation…
I shouldn’t go further, but still… Would it also be so complicated to centralize everything related to licencing (Cubase AND the related contents) in one and JUST ONE tool to ease the licences and download/installed items management ? 
All this said, and this is my remaining concern, I would still like to have reports/explanations about this one…