Amending List content (deleteing & renaming) - Effect on previous created work items/reporting

Question

Hi, 
I going to be changing the naming and structure of support groups and incident tier groups. I'd like to know what effect this has on all previously created work items and the reporting on those work items. 
In addition, I am also adding list items to impact and risk list to change incident prioritisation settings and want to know the effect it will have on the live work items and historical work items.
Cheers,
Pete

Answer 1

When you're thinking about editing enums in a live system, it's important to know a little but about how they work on the back end. 
each enumeration entry essentially breaks down into a GUID and a display string; there are other properties, like Parent, Ordinal, InternalName etc, but those don't come in right now. when you use a console form to look at an incident, the control reads
the GUID that is stored on the incident in the property in question, and then goes and collects the list that property references, and shows you the correct display string for your language. the specifics are not important, but the fact that what's stored
on the incident is that system generated GUID is. when you edit a list, each entry has a unique GUID, that can't be changed or seen through the UI.
Adding a new entry will add a new guid and a new display string. this option will be available the next time the form is opened. this works pretty much the way you would expect it to. 
Renaming an enumeration keeps the GUID the same, but changes the display string. any incident with that GUID will see the new string, and the data warehouse will be updated with the new string, so
historical incidents that had that value will now have the new display string.
Deleting an enumeration will remove the entry from the list and break the mapping of that guid to the display string, but does nothing to the incidents that have that guid on them. the net-net of this is that any incident with that guid
will have a value (i.e. not null) but it won't have anything to show to analysts, so it will look like it isn't set. the datawarehouse keeps those mappings even after they are deleted, so removing a mapping will not affect reporting. 
the end of this is that you should re-classify any items before deleting an enumeration, you should only rename an enumeration if that new name makes sense for all work items for all of history. it is possible to reorder enumerations while keeping their
GUIDs, but it requires some XMLing. I've posted about it elsewhere in this forum, so let me know if you need a link