If they are 14 AWG, then regardless of whether they're flat or not, they should be good up an approximate 50ft. run. I've been using a run of 14 AWG flat wire of about 40ft. to my rear surrounds that have been powered by my Yamaha RX-V2500's internal amp and they do just fine.
The real question I guess would be why do you want to replace the existing wiring? Unless you're experiencing any issues (receiver/amp triggering auto-shutoff, audible compression/distortion at higher volumes, etc.) or the existing wiring is beginning to show visible signs of oxidization, I'm not sure you'll get much benefit out of the change... -TD