dx itself is defined to be always fully backward compatible.
hw drivers on the other side can be buggy and shite. nvidia doesn't have any really good drivers since ages out anymore.
but installing dx9 should not hurt any application wich used some pre-version of dx.
there is only one possibility for a program to fail, when dx9 is installed. that is, if it checks at the start time the dx version and wants to have say dx8 installed, instead of wanting at least dx8. thats what 3dmark2001 had, so you had to patch after you installed dx9, because it did not found dx8 anymore, hehe:D
"take a look around" - limp bizkit