How to know when mwan3 does a changeover?

I tried with /etc/mwan3.user log but everything is "ifdowns" "ifups", "connected", etc.
I only want to know a simple thing, similar like this

Time Changeover to WAN A
Time Changeover to WAN B

The mwan3track component uses hotplug events for various interface state changes. The main ones mwan3 use are documented here: https://openwrt.org/docs/guide-user/network/wan/multiwan/mwan3#alertsnotifications

You can query these hotplug events against specific interfaces to know when it goes up or down and run anything you'd like in /etc/mwan3.user.

How mwan3 switches WAN interfaces is down to mwan3track and the method configured on each interface to determine if the connection is up, most commonly this is using ICMP to one or more hosts. If the threshold of marking a host as down (default is 5 ICMP failures) is reached, the WAN interface will be considered down.