Kog'Maw will be getting an update to his model, adding a bit more texture and it also looks like he's been thinned down. The Armor/MR shred icon for his (Q) Caustic Spittle has also been updated.
While the changes modernise Kog'Maw a bit more, it is focused around clarity for gameplay; primarily for missile widths and hitboxes. He is also gaining a more significant purple glow to help indicate his (W) Bio-arcane Barrage empowered state and his Passive.
Due to a lot of Kog'Maw's skins being 520-975 RP, they have only had small updates to the (W) clarity, some with new colours. Any skins that are less than 1350 RP are not given any unique VFX.
Since his most recent skins have already been made to meet the changes, they've just been tidied up with the hitbox indicators subtly added. Lion Dance Kog'Maw was 1350 RP, so has been given a unique (E) Void Ooze effect, whilst Pug'Maw has broken pieces of balls added to his Ultimate.