Well, I think they look exactly the same. The only difference is that they are highlighted at some parts of their body compared to the original design and that can't possibly be their mega since a mega would have been a bit more extreme.
I agree, they may not be mega's but they are alternate forms. Alternate forms aren't all drastic changes. Look at Burmy and Wormadam, Shellos and Gastrodon, Meowstic, and Aegislash. They were pretty much identical body types with very small adjustments to create alternate forms/form differences.
I don't think these are highlights like how Reshiram, Zekrom, Black Kyurem, and White Kyurem occassionally light up.
Only thing highly off is Groudon's Arm. Where original art, the black lines (which should be the glowing lines in Omega) are indented with the red portions above, but the glowing parts are above or equal with the red parts in the Omega Art work. The arm on the Omega Groudon looks a little different as it looks as if there's an extension to complete the omega symbol which should not be able to be seen if Groudon's arm was remain unchanged while in that post. His spikes get a color change from white to blackish, His tail looks like it may be missing a fin spike on the end. They look more spaced out but that's probably an oversight.
Kyogre has noticeable changes. The tail flaps are different. They have a ribbon down the center of each one. He gains a tattoo on his head which wasn't there before and that tattoo joins with lines from his already existing spots. His two major fins had alterations to the symbols. Where as they were like Nazca lines before, they are more filled I guess you can say. The artwork is similar, but they aren't 100% the same pattern and its a different kind of filling than with Groudon. Kinda looks like he has plasma turbines on his fins.
They are definitely form changes, though this doesn't particularly mean much as there are many aesthetic form changes even with Gamefreak insisting that the form changes are stronger in power.
