Hi
If you are totally changing the IP range then yes, there will be downtime. But you don't need to renew/release. As clients will face interruption anyway you can simply disable and enable the WLAN at once forcing everyone to reconnect.
Now if you are only adding one more interface to a interface group and will not changing gateway or mask, then, there will be no downtime.
-If I helped you somehow, please, rate it as useful.-