This has probably been asked and answered before - but I couldn't find it.

I am currently holding a four segment award ticket from DTW to JNB. It looks like a better routing has become available going outbound from DTW to JNB at the "low" level in Business Class. The segments I have coming back are no longer available at the "low" level. Of course, I ticketed this quite a while ago when they were available at the "low" level.

Can I now change just the outbound segments or do ALL segments have to be available in order to make a change (as if I was booking a brand new award)? I am a PM so there will not be a $150 change fee.