I've heard some rumors regarding Winter War arc some time ago - that its going to be battles in King's realm or the final storyline. But what I heard was a while back - like 9-12 mths ago.
Is it still coming up or has it changed and any updates on it?

I think that we're supposed to be in the Winter arc right now, and that they just moved the war earlier to have the 'surprise effect' I guess.

Since both sides (Soul Society & Hueco Mundo) have brought their highest ranking members.

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I thought its when Aizen actually managers to enter King's Realm....

No...

Soul Society made a prediction, but Aizen decided to go against their predictions, so it shifted to the early time were the manga is at right now. You do know that right now, Aizen is trying to enter the King's Realm, right?

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I know that, I just thought that Winter Wars was the name of storyline once Aizen actually enters Kings Realm.