Old thermostat to Hive thermostat
You can do this, but you'll lose some fancy features

The rewiring you describe (from using the ABCD commlink to using the standard 24VAC inputs) should be supported by your control board.  However, there are downsides to this, namely that your system will act as a good two-stage forced-air system, without full command modulating functionality or the ability to display feedback (such as fault codes) on the thermostat.  So, go ahead, as long as you are cool trading these communicating functions for the Hive's "smarts".