If i Buy one pack do i get discount on another if i decide to go even higher?
No problem at all. As much as I want the Master Inquisitor pack for the physical goodies just knowing that down the road I'll be able to upgrade from the Interrogator pack is good. (plus getting the smaller pack first will make upgrading later less of a hit on the wallet)
Any idea when it will be possible to upgrade packs with discounts in the way you described?
Yes, your previously purchased edition's price will be deducted from any higher pack's normal price label, for example; if you purchased a "Throne Agent Pack" and you want to upgrade to the "Interrogator Pack", your Upgrade Pack's price will be $40, or to the "Master Inquisitor Pack" the price will be $260, and the "Grandmaster Pack" the price will be $960, and so on with the other purchased tiers... : ]
I think his question was that if he spends $40 on a pack now, will an upgrade be -$40? - One would presume but it's a reasonable question :D
Not currently no, this is coming in the near future though last i heard.
As IXXOLOS said; currently no, but in the future yes, there will be "Upgrade Packs" in our Store, if you wish to upgrade to a higher tier edition.