Qualifying fares need to be K class or higher for upgrades, the hierarchy is as follows Y, B, M, H, Q, T, K.
Fare classes Y-Q are 100% mileage earning, T-K are 75% earning.

Importantly not all of these are upgradable as there are various Fare Basis for each fare class, so you need to check the terms very carefully or contact your local THAI office.

For example, I wanted a fare AKL-BKK-ZRH in a fare class I could apply my GUP to. The website provided a Flexi-Saver fare (T and K class), so at first glance this looked suitable. However, checking the fare conditions on the 'more details' link for FlexiSaver, the fare was not upgradable (the fare wasn't a 'sale' fare or part of any special promotion).

I needed to call the local THAI office and pay a much higher prices for a K fare with an upgradable fare basis (around $600 NZ or $US 400 more).

So in the end:

FlexiSaver K Class KM3MNZE $1750 Cannot use GUP (fare as sold on website)
FlexiSaver K Class K1LENZE $2350 Can use GUP (fare basis only avail from THAI office)

The slow rise in YBMHQTK fares, and increasing restrictions on the exact fare basis have also degraded the ROP benefits.