mdlTag_setSetDef will allow you to rename a tag set if you can find a way to call it...
You can freely change the tag set name, the tag elements themselves don't refer to the tag set by name, they store a dependency linkage with the element id of the tag set definition element.
I have no idea what the VBA-COM layer is doing (I've never used it)...changing the tag set name should be a trivial operation. The tag set name is stored as a string linkage with key STRING_LINKAGE_KEY_Name, if all else fails maybe you can just read the element descriptor for tag set definition element and update the string linkage directly...
-B