I upgraded my Z110BT with the new software today, and I'm not a big fan of how the XM interface looks like now. I prefer the Z110BT look instead of the Z120BT new look. Is there a way to revert back to that? I thought I saw a thread on this same issue when I was lurking around a few months ago, but I couldn't find it again.

No.

Is there any way to downgrade back to the original version I had? I'm not liking this interface at all.

Not at this point.