Why isn't his upcoming update not considered a full visual and gameplay update? I mean, he is getting new: model, abilities, animations, voice over, splash art, login theme etc; just like every other VGU. Can someone explain, whats not changing? 
The thing is that the visual update itself is more akin to the scope of Fiora or Shen according to Riot, as his rig will be similiar. To put it simply, he's probably not on the level of your average VGU. The things I've read just imply that he is a bit more muscular and wields a greatsword (simply put he wields it with both hands). Of course there's gonna be a new voice over added to all of that, which almost puts him at the level of other VGUs if it weren't for the fact that visually the update seems to be a lot more minor compared to other champions, which probably has to do with the fact that Aatrox himself isn't visually outdated, just a bit of a missed opportunity in terms of what his overall theme is and of course needs to be visually a bit more akin to Varus and Rhaast. Varus has an excuse for looking a bit visually different from the Darkin because he hasn't fully corrupted the body of Val and souls of Val and Kai, but Aatrox did with his host and Rhaast can with Kayn ingame. I think that will also imply that Aatrox sword will have an eye on it just like Rhaast.
and it was really unnecessary to get a new model meanwhile champions who acctualy need it stay forgotten
I see one of two reasons:
Either they are quite busy with trying to finish what they need to before correcting any mistakes, as in the misconception that he's not a getting a full VGU.
Or he isn't actually getting that big of a visual update but still relatively changed to look newer, Reav3 did say that all skin splasharts are going to be different due to Aatrox's base model receiving changes. But it's most likely not going to be a full flushed out new model like Irelia or Evelynn for example.
I really do hope that Aatrox is going to look different for the most part because as of right he doesn't have the features of a being that is described in lore.
"One of the ancient darkin, Aatrox was once a peerless swordmaster who reveled in the bloody chaos of the battlefield. Trapped within his own blade by the magic of his foes, he waited out the millennia for a suitable host to wield him - this mortal warrior was corrupted and transformed by the living weapon, and Aatrox was reborn. Though tales of the darkin have now passed into legend, he remembers only too well the destruction of his race, and wreaks his vengeance one sword blow at a time."
His body features:
Good example of what Aatrox should look like similarly:
https://i.imgur.com/OedpSVc.png
But I still believe that he needs changes to his face as well, but in terms of body looks in this image. I think this is pretty spot on, except I would still keep the wings but have them updated to something better than the current ones.
Fun fact when I was new to league and I saw Aatrox for the first time, I genually thought Aatrox was a female warrior just based off his facial features lol.
It is considered a Full VGU, if you mean the fact that its still listed as otherwise on the Update Schedule, why does that matter at this point?
They are giving him a new model because you can't just take one model/rig and give it completely new animations that it wasn't built for. His appearance is not changing, however.
