Technically it would work, it's actually the opposite - an 8-ohm speaker has higher resistance, and will use less power than the other channels.
If you do just replace the center channel it may sound a bit off especially at higher volumes, there will be a bit less out of the center channel compared to the others. IMHO just replace all the speakers at once keeping it a consistent 8-ohms all around, if you are getting a klipsch center channel get matching L/R main speakers too. Surround speakers don't have to match but if they are 8-ohm that's good enough.