DP: a few weeks ago, booked the following for 06/24/2023 for 93.5k:

BLR-BKK, TG326, Business (I)
BKK-TPE, TG632, Economy (X)
TPE-SFO, BR28, Business (I)

This morning I got an ExpertFlyer alert for Business (I) availability on TG632. From what I'd read on this thread I'd expected to not be able to switch from X to I without availability on TG326 or BR28 (which have long since disappeared), or face difficulty in doing it with out HUCA, but the first agent I reached was able to do it no problem.