source RPMs and updating

[brian]

I've built my own Postfix package from SRPM and have recently updated 
it. To do so, I used the newer SRPM and repeated the process.

But, I'm wondering if, once a package has been built, can it be updated 
using a pre-built RPM without losing the specs used to build the 
original? Or, will the newer RPM simply override everything according to 
how it was built?

Just to clarify, the original built-from-source package was:
postfix-2.5.1-1.sasl2.spf.fc8.i386.rpm

I downloaded a newer SRPM, built and installed:
postfix-2.5.6-1.sasl2.spf.fc8.i386.rpm

But, could I have simply done:

rpm -Uvh postfix-2.5.6-1.fc8.i386.rpm

... and had rpm take care to install in the same manner as was the 
original SRPM?