Without any insight into the system, it’s only speculation at this point but suspect packs would be unaffected other than the usual 3 month timer on inbox items.
With regards to the method of conversion, that’s certainly a potential headache. Reading through this thread, there is ambiguity with regards to developer statements, but the biggest argument for auto conversion is the potential disruption caused by some players having months of dupes. This could in turn result in a less generous ongoing dupe system so as to not tip the balance.
The flipside is that players have lost out being able to use runes upon conversion and so should benefit from something other than the current rate.
One middle ground option would be to set an auto conversion at a level between the two systems (or at least different to the ongoing new system). For instance if the new system gives 2 jewels for a mythic, then the auto could be 1 jewel (just an example, not trying to equate value). The player could then also be presented with an option to convert at the current rate as well so there is a floor no worse than what we have today.
This means that if a player thinks the one off rate is more valuable that they can get more than just runes and thus a reward of sorts for holding dupes. This may not offset the opportunity cost they lost out on but as nothing concrete was ever promised think this would be a fair compromise. I am almost certain the conversion will not feel generous anyway.
It also means that any new permanent system need not be unduly influenced by the massive hoards. This can be designed with the long term in mind and allay concerns of those worried that the hoarders will impact what they themselves would stand to gain from the new system.
Ultimately, the purpose of the thread overall was to shed some light on the player perspective which is clearly split. Hopefully we will get some thoughtfulness from d3h and a realisation that they really would benefit from communicating both the what AND the why.