You might want to consider trying to whitelisting the existing SSID in Air Marshall so it doesn't think an attack is happening.
https://documentation.meraki.com/MR/Monitoring_and_Reporting/Air_Marshal
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).