Actually Vermin, I think that you will find those command slots will be taken by current Captains moving over to the 400. Overall there are very few extra command slots available over our current number.
MAN based FOs who do not wish to go to HKG (of which there are many) will be sitting in the right hand seat for at least the next 2 1/2 years. There may be a couple of Classic command slots in the immediate future but these are going to dry up completely as they phase out the aeroplane. All future commands at MAN will be on the 400 and will be taken by current Classic Captains or HKG based Airbus Captains.
With a 4% pay rise offer on the table, I don't see much incentive for junior FOs to stay.