Unless something changes, we will absolutely update that skin and others when we work on Sivir. The team is committed to that.
Speaking of updates, quality, etc, here's a close up version of the beauty shots GrumpyMonkey did for Annie's announcement. Been looking for an excuse to get them out there.
(I assume I will mangle their size and have to redo it)
I'm still confused though - from your previous post it made it sound like in general future champion skin VUs will be less of a priority regardless of their exclusivity status (and especially because of.)