AFAIK, you can't use these ISOs as an update source.
As you suggest, I usually make a backup of the config of the current router. Then use the ISO to make a clean install on a new VM (use a fixed size VHD, add 2 synthetic network adapters), give it an unused IP in your LAN, and connect to it using the WebConfigurator and restore the backup config (shutdown existing router before clicking on restore to avoid having duplicate IPs - also make sure to configure mac spoofing on the VM network card if it applies).
You just have to make sure to match the interfaces to the correct virtual network card, that you assign them the same way you had them on your prior router in the VM Hyper-V config. Or you can edit the config XML file manually.
If coming from a pre-Hyper-V VM with legacy adapters, the config file will have it's interfaces named de0, de1, etc.; and it will have an interface mismatch with the synthetic adapters (hn0, hn1, ...), so it will prompt you what interface corresponds to which network port. Or you might have to edit the config using the "Assign Interfaces" option in the console menu.