Step one, before you buy anything, is the existing system 230 volt control, as Hive duel channel, can only work with 230 volt control. Hive is also not opentherm, although to be far the method they use gets around the problem.
The Hive TRV sends a signal to the Hive wall thermostat to tell it to run for ½ hour if it needs heat, so the wall thermostat is more like a hub than thermostat, however this of course only going to work if you have Hive TRV heads.
So yes Hive replaces the UP1 and you retain your tank thermostat but not your wall thermostat.
We have plans, W, S, Y, C sure there is another, and the C Plan has been upgraded over the years, because Hive does not allow you access to the com terminal it is internally connected to line, it will not work on the very early C Plan without a tank thermostat.
The main question is why Hive? I think using the Hive TRV heads it is very good, but without those heads, it is rather a half hearted system, Nest is a little better, but again since Nest don't as yet market their own TRV heads, again not really a complete system, EvoHome is better but you again must use their TRV heads, actually heads and wall thermostat EvoHome can work out cheaper, there is Tado which I know very little about.
So in mothers house I had a problem with sun on the bay windows, the standard wax TRV head was too slow, I bought MiHome Energenie TRV heads with the idea of latter adding Nest wall thermostat, but found it worked A1 with the old dial thermostat so never got around to fitting Nest.
At that time the Hive TRV head was not sold, if what I read is correct, the Hive TRV head will work as stand alone, so maybe best option is Hive TRV head first, then add Hive wall thermostat. As yet not talked to anyone with full Hive system, the theory is great, however so was the theory of how Nest worked, until I moved house and found Nest has removed their support for MiHome so they no longer work together. However reason for Nest was two wires between the thermostat and the heat link was all that was required to keep Nest thermostat charged and send both DHW and CH commands to heat link, and it would also work with old C Plan.
Whole idea of geofencing can be done with electronic TRV heads, don't need special wall thermostat, and programming each room independently only needs a cheap £10 electronic head, also non modulating boilers work very different to modulating, so start by saying what you want it to do. And what boiler.
Big plus swapping TRV heads to electronic type which can be programmed is no wiring to do, really simple to fit, and if your going to DIY the time of year to change is not now, if you make a mistake with wiring then no central heating, better this time of year start with TRV heads, then sure of not ending up cold.