I asked BT in the end and they said yes I can.
I recommend getting that in writing.
As mentioned previously it's entirely at their discretion wether to let you downgrade (and if there are any additional charges for this).
Downgrading will start a new 24 month minimum term again, at the lower package speed.
Edit: removed incorrect info
Edited by j0hn83 (Sun 02-Aug-20 16:51:58)