I am planning a trip to BKK and was wondering if I bought a ticket on DL using cash, could I use DL miles to upgrade from Economy to Economy Comfort and how many miles I would need for that upgrade. I did not see US-SE Asia upgrades on the DL site but probably overlooked it.

Also can you upgrade on just part of the flight say ATL-NRT?

Also how is the 747-400 for such a long haul in Economy? I understand AV systems have been upgraded.