_changeDegree form 2->3 (“up-degree”) will result in a geometrical identical curve, the Degree 3 curve will (somehow) inherit the weight of the CVs. It s the same curve, that will be represented in a more complex CV/Weight/Knot setup. There is only one exact solution - but dont ask me for the precise nurbs-math. Within this calculation, also the weight is somehow inherited.

_rebuild will approximate a new, curve with some tolerance, the structure of the curve is completely independent. For a certain tolerance there will be multiple solutions, see also this discussion about a new algorithm to do this.

here is an example of “up-Degree” from 3 (red) to 4 (blue). (without weights)

the red and blue representation of the curve a identical.

the (P) shows the evaluation at 1/3 using De Casteljau’s algorithm of a Degree 3 and identical Degree 4 curve.