You might want to consider trying to whitelisting the existing SSID in Air Marshall so it doesn't think an attack is happening.
If the existing WiFi solution also has spoof protection you might want to consider disabling that mechanism or doing something to mitigate it.
Otherwise the two will think each other is an attacker.
If they are both bridging to the same VLAN it will mostly work fine. You might have some roaming issues. Don't use NAT mode on the SSID.
I'd keep the migration window as small as possible (aka, swap them out as quick as you can).
Ensure you dont have channel overlap, also worth labeling your SSIDs different names to know which network is which until migration complete. Defiantly ensure air marshal white-lists the old SSID to ensure clients can connect both (as mentioned above).
Do yourself a favor and minimize the time both networks are running. Clients won't always roam as expected between different SSIDs. Once you start, prioritize the Meraki installation so you can turn the older one off asap.