Originally Posted by DeltaFlyingProf
You want to use PMUs for domestic upgrades? Maybe I do not understand exactly the logic of what you are doing because I am not sure I understand what you mean by banking PMUs. Do you mean you have PMUs that will expire soon and you want to redeem them before they expire?
Banking is a technique to book a future flight which is after the date of expiration of the PMU. So i'd book a flight for next summer, on a fare which is both cheap and no change fee/refundable. Therefore the PMU is "stored" so to speak for use later - whether I actually use it for the flight I booked, or if I change the ticket to another flight i'd like to take - say Paris. If i changed, the value of the existing ticket would be applied to the Paris trip, and then the PMU would be moved over. Banking PMUs is probably more useful for those who are doing international segments, since Skymiles now has the unlimited domestic upgrade system.
In short: It's basically a way to keep a PMU in play longer - if you have the cash and forthought to book travel into the next year.