Justy-Plato is right. The only different requirement for HT is a good subwoofer for the effects channel (and maybe another if your L&R have no real bass, like my NHT Superzero's). If you are going to upgrade, spend the $ on a good center channel that matches well tonally with your L&R. And of course at least equally good speaker cable for the center.
speaker degradation question
I know this isn't a brilliant question, but every salesman I speak to obviously tells me to replace my Linn Helix front channels because they are 8 years old. I burned them in properly and they still perform perfectly. I am told to upgrade because they were not designed for both HT and Stereo. Thoughts and suggestions appreciated, thanks.