There's a subtle point here about naming things. I would have to say that an agreement of this nature, entered into before the nuptuals take place, is properly called a pre-nuptual agreement and is always called that. The fact of its having been created in advance of the nuptuals determines its nature (and its name) once and for all, and a subsequent renegotiation or alteration does not alter the fact that it came into existence before [pre-]nuptuals. There are plenty of examples of things which have a name put to them and after the passage of time and changes in circumstances the name no longer describes the thing accurately, but the name doesn't change. [OK Ayleurs, give us some examples!]
As to why one would want to fiddle around with such an arrangement, maybe one of the parties is now inclined to be more generous, or children have entered the picture and convinced one party to do something to benefit them, or maybe a small alteration would fend off a divorce, or, or, or.
