That's been the entire purpose of this thread, there's been loads of ideas on how Blizzard could modify the model. The more rugged lifestyle and beeifer models, living with human culture etc etc.
For instance check this out:
Compare that to this:
The first pic showcases less exaggerated hips (straighter posture), rounder face, slightly chunkier model. And obviously a more human cultured hairstyle. All of that can be explained by "assimilating into other cultures" as Ion stated.
This is lore-wise backed up as everyone and their mom knows that High Elves have been living with humans, people even meme about them as the "human potential elves" or what not.
And Elisande, an NPC created and voiced by Blizzard employees, even points this out in an attempt to take a jab at how the High Elves live today.
That's plenty, there's nothing refuted by Blizzard (as Syegfred up there likes to state) that an Allied Race needs to be separated by a millennia or centuries in order to change appearances.
Find me where Blizzard refutes this (as Syegfred likes to use this reasoning too) and then you'd have grounds to say "there's nothing Blizzard can do" with changing the appearance.
EDIT: Like I said before, Blizzard's got variations of lean/athletic models with the Night Elves, Blood Elves, Nightborne. As well as different variation of Beefy models with Humans/Draenei/Orcs/Tauren/Kul'Tirans, etc.
It's silly to think that there isn't another variation of lean/athletic appearance that High Elves could get (which is supported by lore) when we have 5-6 different variants on "beefy" models.