The "Hubble Origins Probe" (HOP, what's been referred to here as "HST
Mark II") would use a new lighter-weight mirror, presumably to allow a
lighter structure and accommodate a cheaper launcher (Atlas, not
Shuttle). The instruments would also be new: the ones built for the
now-doubtful HST repair mission, not pulled out of some museum exhibit.
See http://www.pha.jhu.edu/hop/

One billion dollars is what NASA now says it would charge against its
budget for an HST repair flight, which is just one more way for NASA to
try to kill the idea. Weighing costs and returns (and ignoring the
small incremental risk to the Shuttle itself), to me the HOP proposal
looks better than HST repair.