First I cannot think of any reason you cannot upgrade. Even if you are heavily modified the newer versions have features that might make customizations un-needed. Anyway here is your answer.
Resolution ID: 404872
Error 47 SOWUAE 7590, F=SOX, in Daily Sales Reports/Updates
Entry Type: Informational
Product: Sage MAS 90 ERP, Sage MAS 200 ERP, Sage MAS 200 ERP - SQL Server Edition
Application: Sales Order Processing
Version Reported: All
Subject:
Error 47 SOWUAE 7590, File=SOX, in Daily Sales Reports/Updates.
Possible Resolution:
Note: Use caution when working with this task. Always make a backup first. If necessary, seek the assistance of a qualified Sage Software business partner or Sage Software Customer Support analyst.
1. Reinitialize SOX (Open Order Detail Sorted by Item).
2. Rebuild Sales Order sort files.
3. If Error 47 SOWUZA 1080 occurs, see Error 47 SOWUZA 1080, when rebuilding Sales Order sort files.
Note: Analyzing and relinking SO2 during an update might cause SO2 records to change, causing additional issues. Contact your Sage business partner or Sage Customer Support for assistance.