Hi Burr, yeah I'm pretty conservative about modifying the model. If I ever do make something that would involve making major modifications to the model I would certainly also have some way to disable that.

It is normal for some kinds of things to be altered in certain ways in very small distances though, for example when joining 2 surfaces it is normal that they will join together to get one common shared edge even if their original edges are only within 0.001 units (or slightly larger in the case of join) nearby each other, rather than only joining things that are 100% completely exactly the same at a common edge.

That very small level of modification is normal with NURBS solid modeling, since many times surfaces can be created by a kind of adaptive fitting procedure that refines a surface result until it is accurate to that 0.001 units tolerance value.

So when I find ways to improve a model but keep it within that tolerance level of accuracy, that would be a more normal kind of alteration.

- Michael