Thanks Kpelt for your answer.
Indeed you can't change to the whole domain and I was afraid of having this answer that the account had to be along with the users. That is leaving me with a problem as we have 3 top OU. One for each of our site and of course redisigning the AD is out of consideration and would bring way to many problem.
However I don't understand why there is an option to search for different OU if the users can't be authenticated afterwards ... Lucky for me this is just a POC but for sure first thing I do as soon as I have bought the license is open an SR with support.
Cheers
Seb