Reorder Method - reverting back to economic quantity after being changed to NONE

Sage MAS 90 Extended Enterprise Suite (Version 4.40.0.5)
I manually changed the reorder method to NONE from economic quantity for about 20 items that we are phasing out and will no longer stock. The following day, seven of these items automatically reverted back to economic quantity. The only transactions we noticed on these were that NEW sales orders were processed, which is fine since we want to exhaust the stock; items that had existing sales orders prior to our change to NONE did not revert back. However, we want the reorder method to stay NONE, as this affects how it is treated in SIA and is also a flag in some of our reporting. What can we do so that it stays NONE after we change, or if this by design, what is the reason? Thank you very much.