Hi Rich,
Just to elaborate on Syamantak's (excellent) comments; some client devices save the AP BSSID as part of its WLAN (SSID) profile. A BSSID is generated from the AP's base MAC address, and changes for each SSID, including an identical SSID on 2.4GHz and 5GHz.
So, in the instance where you swap-out an AP (even a like-for-like model) with an identical configuration, some clients won't automatically reconnect because the BSSID will have changed.
In my experience, it tends to be low cost/low performance client devices that have this nasty habit and thankfully it looks like you've managed to avoid this issue.
I hope that helps.
Thanks,
Darrel.