If you have speakers in the doors then the speakers in the dash are only tweeters.
Once out you will see that despite the hole in the dash accepting 10cm speakers, they are only tweeters which are about 1 inch big mounted in a 10cm plastic pod.
If you put full range 10cm speakers in the dash and connect to the existing wires they will still only produce treble as the factory crossovers will only send high end frequencies to them.
My point is, weather you put tweeters or normal speakers in, they will sound the same as they are still only receiving the same high end signal from the crossover, so dont bother, just put tweeters in.
A way round this is to amp either the dash speakers (with new cable) or door speakers, you will then be running a 4 speaker system up front plus your rears but you will loose the tweeters.
Personally I wouldnt bother, just stick to ugprading the door speakers and dash tweeters in the original locations, once amped up you would have all the vocal response you could possibly want, add a sub for bass and it will sound spot on.