When knowledge about a configurable system grows and user requirements change over time, the corresponding knowledge base of the configurator software has to change as well. That will also affect existing configurations based upon that knowledge base. The knowledge engineer must find the appropriate balance between organizing the different versions of constraints in the knowledge base and upgrading configurations, thus making them consistent with the new constraints. Both of these alternatives have drawbacks and cause effort for knowledge engineers and users. A formal solution for this problem is still missing.