1) only one upgrade allowed per sector, only to the next highest available class
note: if you can upgrade from Y to J (initial aircraft has no PY), then an EOQ happens to an aircraft with PY, congrats you've struck gold!

2) no double upgrades allowed whatsoever for any sector, even if you have an airport upgrade coupon

3) no upgrades allowed for any sector(s) redeemed via Asia Miles (cash+points flights can be upgraded via Miles, but rarely would a sane person use cash+points)

4) upgrading via Asia Miles is subject to the same availability as if you were redeeming an award outright
I.e. if you can't redeem a certain class for a certain flight, then you can't upgrade to it either
(Edit: this point has been contested by another long-time FT user)

5) eligibility for upgrading is dependent on your initial fare class


Hope this clarifies things!