If I understand correct, the upgrade would involve switching the panels which is a major change. Not sure though if I would be upset, you pay for the full HD with lower contrast ratio and I am guessing when you compare D9 HD and D9 Full HD it comes down to personal preference rather then significant difference.
I am also thinking that B&O will update panels at least once a year going forward, so every time you turn around there will be some sort of improvement that in the end is minor for your day to day viewing.
The 50" screen is somewhat borderline where full HD makes sense, not sure what you use to feed it but most cable TV, DVD and what else there is won't live up to it.
Cheers
JK
BS9000, BS2300, BC2, BL2500, BL3, Bl2, BS1, BV8, BC4, A8