Once upgraded, make sure that the APs are in the same broadcast domain. If the APs are connected to a layer 3 (routed) switch, then you will need to configure them for layer 3 adoption. In doing so though, you will not have seamless roaming between the APs.
Recommendations:
1 - Make sure they are both exactly the same model
2 - Make sure they are both on a more recent firmware version
3 - Connect to layer 2 switch
4 - If layer 2 switch not available, configure the AP for layer 3 adoption
Thanks
Christoph S.