Key Flexfleld

In E-bus 11i (yes still on 11i), I currently have the Asset Location Flexfield defined as 3 segments.

I'd like to extend this to four segments without invalidating the existing data

I understand this might require a complete new setup of Fixed assets (Book, Book controls, Cat, Location, and then the migration of asset data) and I have created 4 new Value sets for the flexfield, however when I try to create a new Location KeyFlex (Setup, Asset System, Financials, FlexFields, Key, Segments (Flexfield title "Location Flexfield")) it doesn't allow me. I can only change the current one (which will invalidate all the existing locations).