A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 102,617
EC is not an upgrade, they're just special seats that certain elites can reserve for free (as well as those on very expensive tickets, IIRC FULL Y only) while others can select for a credit card payment. It is not possible to use miles for this currently, although you could use miles for the ticket and pay with money for EC seats.
Yes, you can purchase EC for only one segment. TPAC is probably more expensive than NRT-BKK.
You can use miles to UPGRADE from coach to BE if you buy a Y B M ticket, which is very expensive.