I respectfully disagree. I can change any level you name, and it will still work.
EDIT: just made a quick testing on several levels. I removed and added creatures, added some simple triggering options dependant on level time, changed some surface through triggers, created some new action points.
All levels were finished without a flaw. I was experimenting on levels 1 2 and 6.
It also works with other levels, I had a fully modified campaign, and as far as I remember, all levels were working perfectly fine.
I will write a guide how to edit campaign levels so there would be no confusion about this topic any more.