I have owned both the k550 and the he400. I personally feel like the he400 is a pretty significant sound quality upgrade if you like dark/warm headphones with amazing clean tight visceral bass (great extension, textures and detail). The he400 bass easily beats the akg k550. I also prefer warm sound sig, the he400 is amazing at edm (which was a genre I felt the k550 was a bit lacking in, so could work out well as complementary headphones). He400 are the entry level ortho headphones so that is kinda cool. Weight & comfort was never an issue for me.
However, note the akg k550 is a great choice as well if you prefer a brighter treble focused pair of headphones. In that case, I would recommend upgrading to the akg k701/702/q701 or the sennheiser hd 600 as those lean towards the brighter side. Akg 550 has really sharp bright treble that really illustrates detail and clarity. Amazing sound for closed headphones.
The closed akg k550 has better isolation and great sound stage for closed headphones, but its sound stage cannot compete with open headphones like the he400, hd600/650, or the k701/702/q701. All those open choices have better sound quality than the akg k550.
I personally think the akg k550 is one of the best closed headphones up to the $400 price range and I think that the he400 is one of the best open headphones up to the $500 price range (you can get em for $300 nowadays). There r a lot of great options out there so it just depends on what sound sig you r looking for.