What I'm worried for the MM... loyalists (not sure if the best term, it sounds ironic but isn't) is about eventually finding out how this change was handled internally. I don't think we ever will (or should), honestly.
It's a different band, but Slipknot has gone through many dramatic lineup changes in recent years (letting go of Joey Jordison, Chris Fehn and Jay Weinberg, losing Paul Gray (RIP) and Craig Jones) and these events have been explained in much vaguer terms than the MM->MP switch. A lot of fans also feel like they are owed more details and a proper explanation, and whenever those details have surfaced outside of the band's control (like the court documents for when Chris Fehn sued the group), it's only made the general picture worse. The band is weathering through the bad press and ultimately the backstage stuff doesn't really matter to anyone on the outside.
With DT, we have the benefit of all parties opting for a cordial route. I can absolutely understand the disappointment or even heartbreak of seeing a beloved lineup change, and this sentiment should be respected. What I'm saying is this might be the most details we ever get about the switch, barring a more inquisitive approach in an interview or personal interaction that could not only put the band members (current and former) in a delicate spot and at the risk of being misinterpreted, but wouldn't bring anything good to the person asking the question or to the people waiting for these answers.
My bet (and this is personal, speculatory and whatnot) is that the change, while exciting for those who wanted it, took place in a pretty boring and safe fashion. The points in time where JP got the idea, started talking seriously to MP about it, brought it up with the rest of the group, and MM was informed are ultimately meaningless.
Plus (and this has nothing to do with who he was in terms of active contributions to the group), I think Mangini's happiest time in DT seems to have been before he entered the band, in the documentary. He's an absolute pro and did everything the band needed to keep going (I especially like that he helped with songwriting with Paralyzed, Room 137 and The Alien, to mention a handful of examples), but I've always felt like he's followed by a dark cloud when talking about his work in the group. Not in terms of personal interactions with the members, but the whole ancillary structure – the public perception of his drum sound on the records, the "precision vs. feel" debate, being called upon to only play drums on The Astonishing (where, mind you, JLB and JM weren't also invited to the songwriting process as far as I know)... I've seen too many interviews where it felt like he was making excuses for things that were decided for him. He was always enough of a team player to take that and the (often unfair) criticism in stride, but (again, just speculation) it might be that he took the call of "we're getting MP back in" less like "oh no, my life's work! How dare you shitcan me?!" and more like "oh well! Time to [keep doing!] my stuff on my own then".