I don't know the OP's shops reasons, but a few years ago we upgraded an old VMC. It had a few problems, but it's new owner worked those out and has it running like a champ now. We upgraded mainly because of the old control (DX-32) that while sufficient for our needs, getting machinists to use it was like pulling teeth because it was "different." The new machine is Fanuc which matches other controls, has a higher speed spindle, faster tool change, as well as it came with some other add-ons like tool and part probes. Newer mill also had more support while the old one was technically obsolete.
Sometimes replacing machines is more politics than anything. Management has to pick and choose which battles they fight with staff because just canning someone for not doing things exactly how you like isn't always fair. A new machine with new paint can do wonders for moral and motivation. Instead of managers trying to sweet talk operators into getting 100% utility out of all the old machines, the operators themselves start looking for excuses to use new features and improve old programs with the new machines. People see new tools coming in the door and think the company must be on the right track. It's not a fix for staff issues, just a tool in that helps.
All that said, I don't know that a new Haas is an even trade for an old Mori? Kinda like trading in an old BMW for a new Toyota. I really wish Haas put more into their machines, but that's perhaps a different conversation for a different thread.