It would be easier and probably better to try this:
Change your existing departments/categories. You can rename them where possible and add new as necessary.
If you want to make wholesale changes to your ILC scheme, etc. you can use a query to extract the data, make the changes in Excel then use another query or set of queries to make the current data reflect these changes.
For example, you buy Medium Widget #1234 from vendor Widgets Inc. You initially used the vendor ILC of 1234 as your ILC but would now like to use the UPC code or perhaps something like WID-1234. You have successfully renamed the category from GADGET to WIDGET, but would like to change the category from LARGE to MEDIUM, because you initially had categories of SMALL & LARGE but have decided that MEDIUM is a valid category.
First, export the results of this query: SELECT ID, ItemLookupCode, Description, DepartmentID, CategoryID FROM Item
Open the results with Excel. BE CAREFUL. If you have leading zeros
*ANYWHERE* in your data, you will probably lose them. This could be VERY BAD. If leading zeros are a concern, open the data with your favorite word processor/text editor then copy/paste into a text-formatted column in Excel.
I could go on an on about this, but hopefully you've got the idea by now.
Maintaining your item/customer history and "starting fresh" are more or less mutually exclusive.
Tom