As I often do, I am working on a project where I am importing lens surfaces and rays from an optical design program (ZEMAX, in this case). To make certain that all optical components are in the right place, I create mates between model components and surfaces in the imported IGES file.

If anything changes in the optical layout, I must re-generate the IGES file, and, even if no referenced surfaces have moved, all the mates break, because Solidworks doesn't recognize them as being the same.

Correcting the mates is easy. The problem is that the rebuilds after each mate correction take a long time and also pose the risk of completely screwing up sketches based on the locations of these components. What is particularly dangerous is that Solidworks 2009 seems to arbitrarily change some of the dimensions in some of my sketches---so instead of getting a broken part, I'll get a part that looks fine but in fact has had a few key dimensions changed. This is particularly horrific when it's happening inside a lens tube---if you don't section view everything vigilante style, who knows what Solidworks could be "designing for you".

Is there a way to suppress these rebuilds, or perhaps a way to directly substitute surfaces (as you can substitute components, for example)?