It would be really useful if we were able to update the categories available and for the change to reflect in all previous tickets.
For example, a ticket is categorised as
3rd party system
ABC product name changes to XYZ
Currently, if I change the category in the field templates, it removes the 3rd level "ABC" from any existing tickets and does not apply the "XYZ"
At the very least it should retain the "ABC" for reporting.
I am presuming that this is actually quite a big change as it requires the underlying data to be relational whereas it is currently just free text
I've been working in restructuring my categories as well and I am having the same issue.