I think the Amazon ELB is able to call a healthcheck URL which needs to indicate the status of the server through a response code. But just switching the origin of the ELB is not the solution. If the other server would still run ore come up laterwards and runs in parallel with the failover you have 2 block products at the same time which will leat to slot battles between them.
In my opinion you do not need a LB. Both machines can be running at the same time, but the standby needs to be configured to be a started as a relay. In case of the switchover it starts as a validator.
I have documented the steps which need to be done in this case and tried as part of my update process. I have not fully automated it so far but planning to do so.