Magnus, if I understand correctly, you are booked BKK-NRT on 12/4, but would prefer to fly 11/4 in the evening. You are asking if you can 'standby' for an evening departure to NRT on 11th instead of your booked flight on 12th, right?
Here is the current availability for the 11th BKK-NRT:
[KVS Availability Tool 3.0.3/Platinum - Apollo: ITN/US-ARL]
Code:
BKK Bangkok Int'l TH [VTBD]
NRT Tokyo Narita JP [RJAA]
FRI 11 Apr 2008
Carrier Flight From Depart To Arrive A/C St Availability
--------- ------ ---- --------- ---- --------- ---- ---- --------------------------------------------------
TG 676 BKK 07:35 NRT 15:45 747 0 F4 A4 P0 C4 D4 J4 Z4 Y4 B4 M4 H0 Q0 T0 K0 S0 V0 W0
TG 620 BKK 09:20 KIX 19:30 772 1 C4 D4 J4 Z4 Y4 B4 M4 H4 Q4 T0 K0 S0 V0 W0
-> TG/NH 6034 KIX 21:25 HND 22:30 737 0 C4 D4 Y4 B4 M4 H4 Q4 K4
TG 622 BKK 22:00 KIX 05:30 +1 773 0 C4 D4 J4 Z0 Y0 B0 M0 H0 Q0 T4 K4 S4 V4 W0
-> TG/NH 6032 KIX 09:55 +1 HND 11:05 +1 772 0 F0 A0 C4 D4 Y4 B4 M4 H4 Q4 K4
TG/NH 6004 BKK 08:15 NRT 16:20 777 0 C0 D0 Y0 B0 M0 H0 Q0 K0
TG 640 BKK 22:10 NRT 06:20 +1 773 0 C4 D0 J0 Z0 Y0 B0 M0 H0 Q0 T0 K0 S0 V0 W0
TG 642 BKK 23:50 NRT 08:10 +1 772 0 C4 D0 J0 Z0 Y0 B0 M0 H0 Q0 T0 K0 S0 V0 W0
TG/NH 6002 BKK 23:55 NRT 08:05 +1 777 0 C0 D0 Y0 B0 M0 H0 Q0 K0
TG/Japan 6934 BKK 23:55 KIX 07:20 +1 767 0 C0 D0 J0 Z0 Y0 B0 M0 H0 Q0 T0 K0 S0 V0 W0
-> TG/NH 6032 KIX 09:55 +1 HND 11:05 +1 772 0 F0 A0 C4 D4 Y4 B4 M4 H4 Q4 K4
TG 672 BKK 23:59 KIX 07:30 +1 772 0 C4 D4 J4 Z0 Y0 B0 M0 H0 Q0 T0 K0 S0 V0 W0
-> TG/NH 6032 KIX 09:55 +1 HND 11:05 +1 772 0 F0 A0 C4 D4 Y4 B4 M4 H4 Q4 K4
those TG evening flights (TG640 and 642) look booked solid at the moment, and my guess is that they will stay that way. your dates are very close to the Thai New Year (Songkran), usually celebrated 13-15/16 April. A lot of Thais travel during the period 10-20 April.
It is possible to travel on an a 'standby basis' for an earlier flight (depending on fare class), but in your case, if there are no seats (unless there are some 'no-shows'), then you will have to return to town for the night - which could be a hassle. It might be better to 'waitlist' for a permanent change to the booking, although it looks slim. Maybe sticking with Plan A (current bookings) might be prudent.
Cheers, TK