I don't think you need to use the CH or DHW off terminals in the hive unit, but much depends on the plan being used, Honeywell has the S, Y, W, C and likely more plans, and the C plan has three versions, where with Nest for thermo syphon central heating you hard wire, with Hive it is a software setting.
As far as I am aware you don't need to rewire anything Hive can even fit on the same base as the Honeywell, but would be better if the old thermostat is removed, but could simply be turned to max.
The big advantage with Hive is the demand for heat system so Hive TRV heads can make the boiler run when required. but this is a software setting not hard ware.