What do you mean by 'disappear'?
So you cannot see the subform records that had the old part number anymore?
I've seen that problem before and cannot recall the solution to it. One possible workaround would be to order your combobox by ActiveStatus first, and then by PartNumber (or whatever) second. In that way the 'old' records would still be in the combo, but pushed to the bottom where they'd be nearly invisible. Remove the "True" criteria in this case, of course.
Do this in the query that your Combobox's Row Source is based on. You will probably have to reorder the columns in your query to put ActiveStatus before PartNumber, unless you already had it that way, so make sure your Description textbox still has the right column.