Well, probably not. Take a read of this:
https://documentation.meraki.com/Architectures_and_Best_Practices/Cisco_Meraki_Best_Practice_Design/...
And not this bit in particular:
"The switch starts a countdown of 20 minutes to allow any other switches downstream to finish their download"
So when you trigger an upgrade of all switches in a network at the same time, no switch will reboot until all of the switches have complete downloading their new firmware - or 20 minutes have passed (which usually means an upgrade failed).