mismatched impedance

Can I hook up 8 ohm speakers to an amplifier that expects a 4 ohm load without damaging the amplifier?

I ask because I'd like to upgrade my surround sound system but can't do it all at once. I'm currently running an older LG TH-9654S htib that came with 4ohm speakers. It was fine for our apartment (low volumes), but in our house the plastic enclosures are really ruining the sound quality for me.

Great! I'm always a little wary about mis-matching impedances because coming from the tube guitar amp world it can mean blowing up the output tranny.

Somewhat of a related question...should I look at replacing the fronts and center, or should I just replace the fronts? It's used about 75/25 for movies/music. Obviously I could spend a bit more on the speakers if it was just the fronts.