As far as I’m aware the main differences between the HM.1 and HM.2 standards AFAIK are mainly in the computer systems. The HM.1 currently uses an outdated computer system designed In the 90’s and with proprietary processors and components the new mission computer systems will use COTS and MOTS components and will be much easier to upgrade. This new computer system should lower costs and be far simpler to apply software or hardware upgrades to as well as lowering support and training costs. The cockpit will also be updated to an all digital standard and a new flight control system will also be integrated. Other changes will be updates to the engine hardware and software, the rotors will no doubt be replaced with the latest BERP blades and other hardware and software updates will be applied across the airframe.

I personally think reducing the ASW Merlin fleet to 30 airframes could turn out to be a poor decision if any submarine threat emerges. I would rather all of the current 42 airframes be fully converted to the new standard and additional airframes bought if the MoD decided to go down the rotary AEW route again.