My understanding is that:
1. yes, I fares will be hard to find or unavailable during the jul/aug, except now that we're entering July there might again be some last-minute, lower fares. Not sure, but I've seen that before.

2. Even if I fare were available, CX will often hold it back from the website, and you will have to call CX TPE. Looks like they're counting on more profit from some pax not realizing "not seen on website" doesn't mean "don't exist" or "sold out".

3. I travel TPE-BKK a lot. Often, early booking will NOT net you a discount, because CX wouldn't release the I fares that early. Instead, booking closer to your travel date might do the trick.

4. CX's "early bird" promotion - at least for BKK - was a bit of a misnomer. In effect there was no special promotion to speak of. My TPE-BKK is the same price whether that promotion was on or not. In my perception at least, it seemed to vary according to ticket class, not the promotion.

6 months ago, I checked CX.com and there was no I-class available for TPE-BKK beyond new year's eve 2012. Travel agents quoted the D fare for all of 2013, with popular periods costing even more. But once we entered april & may, I think I saw the I fare return to CX.com again. So I think CX is quite proactively adjusting fares year-round to maximize profit, like any other well-run airline would be tempted to do.

In any case CX offers a great way, perhaps the best way, to travel TPE-BKK in J. I seek out the I fare, but if you compare CX to say, TransAsia's J product mix (seat/lounge/value/flight time), CX is a great deal even at D fare. TransAsia can basically go (censored) themselves for what they charge.

Corrections, addendum welcome. :P