There is no re-parser for the old YOUTUBE(s) yet, I was working on it but I'm busy at the moment and I don't know when I will be able to finish my self assigned task. So I think that support can hardly give you a solution for this. Maybe you could make a request in other forums here, or ask for a paid job in the "Wanted!" forum.
[youtube]nnLmzQ6TGjs[/youtube]
won’t work, I suspect that format is from the old [youtube] custom BBCode. Edit that to https://youtu.be/nnLmzQ6TGjs
and it works.