IMO, don't worry about it, it will work just fine without the flushing. I have done several cars now by just filling the MC and bleeding until I get clean purple fluid, and they all worked great. The dire warnings about all the seals needing to be new appear to be wrong as well.
Bought a Chevy new in 1980, then converted it to DOT 5 in 1988 after one of the calipers started leaking. When I junked it in 2003 (with roughly 1/4 million miles), it still had the original seals at the other 3 corners.