If you have access to Linux machine, you can run this command-
Disable/delete ZHA integration (make sure HA is not accesing your Zigbee Coordinator)
sudo bellows -d socket://IP:8888 backup > backup.txt (if you are using ZBBridge/other IP based coordinator) OR sudo bellows -d /dev/ttyUSBX backup > backup.txt (if you are running USB based coordinator - make sure you write the correct USB path)
Open backup.txt and change radiochannel=X (X to the channel that you want)
sudo bellows -d socket://IP:8888 restore --force --backup-file backup.txt (if you are using ZBBridge/other IP based coordinator) OR sudo bellows -d /dev/ttyUSBX restore --force --backup-file backup.txt (if you are running USB based coordinator)
Note: the above method only works for EZSP coordinator.
Zigbee network channel 15 is used by default by ZHA but the Zigbee network channel can be changed for devices not yet paired via channel mask in YAML like you done, however note that changing Zigbee network channel for already added devices requires repairing all already devices paired to ZHA. That is, you can not change channel of the devices on-the-fly.