I am replacing 24" double wall oven with a new 30" double wall oven. I know I will have to up my 40 Amp circuit breaker to 50 Amps, but Do I have to replace the wiring too? Presently the cable to the ovens has a #8 on it for 8 gauge. Is this OK?

#8 copper NM-B, or Romex, is only rated 40 Amps, #8 copper MC cable is rated 50 Amps. If you want or need to use Romex, you will need #6 copper.