It is common advice around here to take care in selecting your center and main speakers so that they sound similar. You can certainly change center channel speakers, but it would be easy to pick one that sounds different enough to be distracting. I am sure there are some good, or good enough, matches as well. But I don't have any specific advice on that. I do have another option you might like to consider...
I have an LCD and a TV stand, too. I added an LCD mount to the stand so that I can put the center speaker on top of the TV stand.
I did a DIY (but REALLY easy) mod using an LCD mount from Bello. My stand is from another manufacturer. I bet the Bello mount would work with a lot of different stands, and it isn't very expensive, either. Here are some pics:
http://forums.**********/showpost.ph...10&postcount=5
^ replace ******** with "a v 1 2 3 . c o m" (minus the spaces)
With some of the DIY details...
http://picasaweb.google.com/maxcoope...HybridTVStand#
Of course, you could also just buy a whole new stand with an integrated flat panel mount. Since flat panel TVs have boomed in popularity, such stands have also become rather easy to come by. Costco and Wal-mart both have them, for example.
-Max