It might be worth chatting with Yale but I have a feeling that you could change your existing HSA6400 hub to a Yale smart home hub and keep all your existing sensors alarms etc. This connects via ethernet to your router hub and allows full control of all your alarm functions via your mobile. You can even add cameras if you are getting paranoid.
Yes, that is an option as I have done exactly the same - switched from the Control Panel based HSA6400 system to one with the hub in it - it's the one contained in this kit..
https://www.toolstation.com/yale-hsa-app-enabled-ala...
I was lucky enough to find just the hub on sale from the eBay shop Yale has, but I can't see any currently there. It has to be the HSA compatible hub, not the Smart Hub. You can use the app to solely arm/disarm it but I also bought the small keypad they sell to have that option too - more for my kids than myself as we (wife and I) have the remote key fobs.
If you install the keypad it has to be positioned
before any point where you would pass a PIR sensor as the way the PIR sensors can be configured differs slightly in the app compared to the Control Panel. In the CP a PIR could be set as an entry sensor to allow a path to the CP, on the app all PIRs are instant burgler activated when the system is fully armed.
As I upgraded the hub before I had FTTP as an option I never tested if the Control Panel worked on the VoIP service I now have - Digital Voice with Zen 500.
The hub just connects straight into the router by ethernet cable and all the sensors remain the same. You just need to remove the Control Panel from the system (siren) and add (pair) the hub to the siren.
You then use the app to control the system and can arm/disarm etc. from the app. It (the hub) can also integrate with Google Home and can be used within routines. e.g. I have it set to part-arm my system when I run the Good Night routine and fully arm it when I run the Leaving Home routine.