As Merc was derived from DIKU, and several others from there... I was curious what gives the (team) the right to rename the base

IMO you can change the name with only a tiny bit of a change. AS LONG as you keep the liscence that the codebase came with, and you probably SHOULD still give credit to the codebase.

When you should be able to stop giving credit to the original codebase is once it's 100% different. The advantage to not changing the codebase name is people know what to expect with the codebase and it helps them, so IMO there's nothing wrong with not changing the name once there isn't a shread of original code left. AFAIC (As Far As I'm Concerened) you should give credit to the original codebase until there isn't a single bit of code left.

That's all my 2 cents.

Note: I don't know if any of what I said is illegal.

As this is the coders forum rather than the legal one, I'll use coding as the context. However I would first like to briefly clarify the legal point definition, as someone else has already commented on it. Once you've added sufficient material to make your mud a "new work", it automatically becomes a derivative - and you cannot "underive" a mud. It will always be a derivative, unless you delete it and start again from scratch.

From a coding point, however, I generally define the difference between a "mud" and a "codebase" in that the latter is publically available, and once you've made a codebase publically available you should give it a new name to avoid confusion with other codebases. That doesn't mean you can take out any previous credits, obviously, but it does help clarify to other people that it's not the same version as it was based upon.

However there is little point in releasing a codebase unless it adds something worthwhile. Releasing your new "R0X0Rmud", which is a stock Merc with colours added, is not really very constructive.

Hmmm, it should read in the license of the MUD you took as an original what you need to do to make it a derivitive instead of a modified copy. I am not very familiar with any of the licenses used by the often-used code bases though so I can't help you people with exact details.

Numeric qualities are not what constitute a derivative or not. There is no "magic number." Moreover, the numbers in your poll are arbitrary and have no foundation. Why not 43.23458 as a number?

Derivatives are different depending upon the medium. However, a mere trivial variation does not a derivative make.