Upgrading from speakers low in the line is usually pretty good bang-for-the-buck. An inquiry on the Klipsch threads wouldn't hurt to see if someone who has owned those speakers and upgraded can weigh in. Chances are, if you want to upgrade the overall SQ, upgrading at least the fronts will likely be a part of the plan. Upgrading surrounds is optional IMO if they basically timbre-match to the newer Klipsch as they'd likely yield far less bang for buck.
SoM, one worry about the Klipsch is that they are usually super-efficient speakers (>100 dB), thus can easily lead Audyssey to max out with trims on the -12 dB side. Of course, this all depends on room size and distance of speakers from MLP, but many have reported such anomalies in the past. Hard to define a normal room size precicely, Well, let's say it's not a gym! Return policy comes to mind!