Keep in mind as Valg said if the owners of the original expressly
give permission to make changes, then legally the changes are yours.
No matter what you do, it is still a derivative, but a derivative
work is not theirs.

from the diku license:


You are allowed to alter DikuMud, source and documentation as long as
you do not violate any of the above stated rules.

So from that, if you violate the rules laid out in the diku
license, you lose permission.
This part is where I think people got confused along the way.
They assumed because diku credits stayed in all this stuff
through merc and rom etc... that the rights still belonged
to diku.
No they stayed in because to take them out takes away
your permission to make changes. If you have permission to make
changes the changed version is yours, if you don't, it isn't.

Does that make sense?