Originally Posted by
DeathSlam
But I imagine the change is quite expensive as you are 'wasting' half a voucher and paying full whack for First.
Yes it can work out more than expected. However because the Avios cost difference between business and First is so low (and sometimes 0 or negative), then "wasting" half the voucher doesn't really matter.
In my case the cost was around 30k Avios and around £250 in extra taxes.