It's not unethical in the slightest. A full Y ticket is changeable/cancellable/refundable - period. That's the purpose of that fare.

As others have noted, if the Visa appt is only a few days before the flight, you won't be switching from an $1800 fare to a $600 fare - it'll probably be something a bit higher. How much higher depends on how close the flight is to sold out.

I like the idea of booking the $600 fare now and - worst case - you're out the change fee if the trip is delayed. (I assume the trip will eventually happen, right?) That risk seems somewhat lower than the risk that the $600 fare will rise significantly close to the travel dates.