Originally Posted by
MSPeconomist
or buy an expensive coach fare and use miles to upgrade into upgrade inventory, if available,.
You can upgrade to D1 with miles from many coach fares now, not just the most expensive, but the amount of miles required from most coach fares is very high.
Originally Posted by
MSPeconomist
.....or benefit from an OPUP if coach is oversold on your flight but there are otherwise empty D1 seats and you get lucky.
This is rare and should not be part of OP's plan or expectation, even though it is technically possible.