Incase anyone was interested, I had the following response from Grant when asking about retrofitting their new controller to older models:

The Grant Smart Controller is backwards compatible and can be retrofitted to an existing Aerona 3 heat pump system. However, there are qualifying criteria..

If the Grant HPID6R32 ASHP was manufactured after 5 February 2021, then the new Smart Controller can be retrofitted, with the proviso that the existing wiring centre will need to be replaced, additional components added and  potential wiring upgrades implemented. Additionally, the original remote controller will need to be left connected..  If removing the remote control is needed, a new control PCB will have to be purchased.

If the Grant HPID6R32 ASHP was manufactured before 5 February 2021, then a new PCB will need to be installed / system upgrade. It is not possible to upgrade the software on-site so a charge will be applicable.

Currently, there is not an ‘off the shelf ’ retrofit kit solution for the above configuration;  therefore, the controller andadditional components will need to be purchased as individual items and installed as replacements to your existing system controls. The controller system upgrade will most likely require an installer to carry out the work, and would therefore be an additional cost, to the aforementioned components.

As can be seen from the above, the smart controller design is technically a retrofittable product. However, it is not a simple ‘one in, one out swap-out ’ and as such, may not always be considered as practical retrofit for an existing Grant ASHP installation.

As my unit was installed in 2020, as above it's not going to be worth the hassle or cost to retrofit the new controller, as much as I would like to so I have a more user friendly interface and easy access to data.

I haven't bought the Aerona Smart Controller (£700!) but my guess what they mean with 'retrofit' is basically undoing all the weird external switching / timer / wiring centre boxes that Grant's installation instructions told the installer to fit in the first place. The Aerona3 heat pump has some dry contacts for zone valves, pumps and immersion heater, so all you need to do is wire those up directly and then configure the ASHP to use them (possibly with a bit of relay assistance as the contacts are only rated at 250V 1A - I've put in relays for everything).

It's actually a lot simpler than the bizarro rats nest of external boxes they told installers to fit in the first place.  I figured it out by myself and just left all the boxes in place, dropping an additional 8 core mains cable and an extra piece of cat5 to the outdoor unit. I think £1500-2000 is a big overestimate for how much work it is: a small consumer unit with a circuit breaker and some DIN relays/contactors, a couple of cable runs outside, a thermistor in the hot water cylinder and that's basically it.