Works like this for me:
Single OpenVPN road-warrier server instance bound to Localhost.
Port forward on both WAN-1 and WAN-2 to the same OpenVPN localhost instance.
Add appropriate FW rules to allow forward VPN traffic
Separate DDNS entries for each WAN.
Then
In the Client config file, simple add two entries for the VPN host connections, i.e.
remote wan1.ddns.com 1194 tcp-client
remote wan2.ddns.com 1194 tcp-client
Note: I used TCP for my OpenVPN, because UDP didn't work well in my scenario, but UDP should also work.
This way when your two WAN gateways switch from High to Low tier, the VPN clients should reconnect to the second DDNS. Only downside is they will remain connected to the Low tier GW when the high tier comes back online, however when they disconnect & reconnect later they will get the high tier as it's the first in the client list.