OK then, to move the wires for 'like for like' functionality from the BDR91 to Hive would be as below:
Wires in BDR91 N = Hive Dual Channel N (Neutral)
Wires in BDR91 L = Hive Dual Channel L (Live)
Wire in BDR91 B = Hive Dual Channel 4 (Heating on)
Wire in BDR91 C = Hive Dual Channel 2 (Heating off)
The existing link between the BDR's second L and A is made internally by the Hive Dual Channel so it's not needed anymore.
If there isn't a hot water system connected Hive terminals 1 and 3 will be vacant.
I still have concerns about the black wire in BDR terminal C, it will be made live when the Hive switches off the heating. Which will be the same as the BDR does now, so nothing really changes. Except as you can see from the BDR installation manual below terminal C doesn't have a wire in for any of the applications shown.
My concern is what is happening at the other end of the black wire that is becoming 'live' when the heating goes off. It maybe terminated safely at the other end, floating around loose, which could be dangerous, or actually doing something. I don't know.