So I'll just throw this in... I just made the UUA upgrades, and wasn't charged the £150 change fee mentioned in the BAH T&Cs.
But... the chap on the phone (UK call centre, for what it's worth) said that staff were being told to start charging change fees in future when these upgrades are made more than 24 hours after the initial booking was made.
Sounds 'odd' to me, and he did get the Avios upgrade calculation wrong initially, so make of that what you will.