Now, Fluffers, ya knows darn well I loves ya to bits...
It's pretty clear by now I'm not a programmer nor a designer nor a developer and am actually rather significantly techno-challenged. Coding such a thing sounds like it would be a complete nightmare (non-hooved variety).
If the pub 16 or AoS nightmare conversion (hooved variety this time) precedent were followed, then it's likely all evolving/growing/upgrading to super-dragon status dragons would have one standardised across the board set of stats (eg mares received 315hp/515str) and would retain their resists and skills. (Note:- Even that was buggy as hell since some converted critters, like my unicorn, received --- eg, zero in an elemental resist.)
A fresh mare's stats/resists will top a legacy mare's stats/resists almost every time and there's no reason to believe that would be any different with dragons.
Imagine the outcry Smaug/Puff/Fido the 9-10yr old dragon fully quested & upgraded now sucks against the new drags on the block - Or every bit as loud, the screams of injustice from those who do not have a 9-10yr old dragon to upgrade and their new drag sucks by comparison.
The way I see it, tamers are being given a gift of this new creature. Why it is folks want to dictate the manner in which that gift is bestowed upon them is just beyond me.
So many other playstyles & professions are crying out for much needed care & attention (pvp, factions, crafting, resource gathering, etc) that I'm more than a little embarassed that in the face of this really cool parting gift from Wilki that it's some tamers saying, "Well, yeah, cool gift and all, but actually I want more! Yeah, I want a quest with it, too."