Best Way to Replace Default Code?

I want to replace some of the default code which starts out in
etos74\src\bsp\ and gets incorporated into my project.

Fiddling around, I can simply rewrite those files, but…

a) now I’ve destroyed the original master
b) it’s not in my project workspace as part of my subversion working copy
c) my code will get overwritten by updates

So, obviously I want to avoid all those bad things, but I don’t get how I can replace the files and/or routines in the
etos74\src folder or have the build process use versions within my application code instead of the files from
etos74\src\bsp.

– gw