I would do this (carefully, with a known good, tested backup, on a separate computer) using Microsoft Access - Something like:

Open the database in Sage 50, in multi-user mode, as a named user.  Then, using the sysadmin ID:

- Attach to the company database using ODBC

- Import the entire Sage 50 company database to an Access database.  This is another backup / quick reference.  Some tables will not allow access, that's normal and you won't need them.

- Close this database.

- Create a new Access database.  Attach (as opposed to import) to the Sage 50 data.

- Locate, open, and Copy the entire tInvent table (right-click in the upper left corner) into a temporary Excel sheet.  

        - Insert a column, then Use VLookup to arrange the new part numbers alongside the old.  

        - Double check that all old numbers have valid new numbers.

- Locate and Open the tInvUDF table

Each inventory item has an assigned ID number that is the unique key field for all the part item tables.  So, working with the tInvent (main inventory table) and tInvUDF (Additional Information) tables you will be able to:

- Sort tInvent and tInvUDF, and the Excel sheet by ID number.

- Select all the fields (the entire column) and Copy the copy the field sPartCode from the tInvent table.

- Paste into the desired field in the tInvUdf table

- Double-check that the Excel sheet is sorted by ID, then copy the new number list column from the excel sheet into the sPartCode field in the Access database.  

This can also be done with a temporary table and a few SQL commands.  

Or, look into AutoHotKey, it's possible to build a script that will open the item numbers one at a time, click the mouse, and enter text.  I used it last year to fix the result of an attempt by ChimpKey to convert 16000 inventory items from a Sage 50 file into QuickBooks.  (I had to copy / paste in order to swap the Part Number and Description in 13,000 fields, they had them backwards)