This seems like a necessary change. I think going to Unreal is the correct call because it address their biggest Engine concerns. The biggest question will be performance because we've seen games stuggle. It's safe to safe, performance issues a year ago aren't going to be the same 2 years from now with updates and devs getting more familiar with the engine. If Halo studios runs into any issues, they have multiple studios that can help out like the Coalition.
I'm hopeful the studio will get Halo back on track. They have a whole new leadership team in charge with Pierre Hintze leading the charge. The guy seems to know what Halo needs and what the fans want.