COMInterop quirks

I have been working with COMInterop for about 6 months now.  We Interop to a VFP dll.  Today, we added some debug code to the VFP Dll to simply write to a text file.  We did not change the interface.  We tested, figured out a problem, removed the testing code from the VFP dll and then updated the registry (regsvr32 /u and then registered the updated dll).  We did not change the interface at all, only implementation.

That said, I did not do a rebuild on the assemblies that use this dll, thinking the interop dll would not change since the interface did not change.  Well, we are seeing all kinds of 'stub received bad data' on the 1st method call to this VFP dll.  So, I rebuilt the assembly that uses the Dll, and sure enough, the interop dll modifed date was updated to the current build timestamp.  Am I missign something here? 

The interop dll should not change if the interface to the com dll is not changed, correct?  This brings back dll hell nightmare memories.