If that's the case would in-wall M5HP's be over kill for Atmos celling? I have 4 of them for my surrounds.
I assume you are 7.1 or 7.2 right now? What is your Atmos goal, 5.1.4 or 7.1.2 or 7.1.4?
Can’t tell if you are looking to buy additional m5 in-wall for atmos or to repurpose what you have?
If you are keeping what you have for surrounds and looking to purchase additional speakers for Atmos, I suspect M5 in-wall might be a little beefier than what you need for overheads.