It sounds like they're just making the endings a little more different by showing what happens to the galaxy after the Mass relays explode in each one. Unfortunately, that is my absolute smallest problem with the ending.
I see they've also mentioned new dialog that "clarifies" some things(e.g., massive plot holes), but I'm having a hard time figuring out how they could possibly fix the logical beating the Reapers went through with that fucking kid.
And it won't change the fact that the entire final third of the game felt pretty unpolished. The entire last mission is pretty damn bland compared the final missions of both previous entries.
PS: Anyone else remember that bit when you can go around and have a final word with your squadmates, and it gets randomly interrupted in the middle by a totally pointless turret section, as if the designers thought we would nod off if we went five minutes without shooting something? Yeah, that was really stupid.
...Yeah, I'm done now. WAIT, my opinion on the whole thing. Yeah, I'll watch it on youtube. We'll see. Most likely it'll fix a few tiny problems while making the bigger ones that DON'T get fixed only seem bigger. It will result in a massive shit storm (hopefully but probably not smaller then the first one) and I'll have to avoid this site for about 5 months. Short version: It'll be like a band-aid on a decapitation victim.