I understood there to be an extra HUD session if required?
As far as I know, there is some additional HUD training, but it's optional, as not all A350's have the HUD fitted. I don't know the scope of the HUD training, but I imagine it's an extra FFS session. At least that's how things work when transitioning from 777 to 787. Unfortunately I don't have the A330/350 OSD at hand to check it...