Good morning Craig,
Seems to be a missing link and I would open the original SW file and do a Pack & Go rather then a save as, then all you need to do is check the box where you want to replace the numbers and insert your part numbers and all should be good.
One method that is available to re-establish association between files (i.e.: drawing-to-assembly/parts or assembly-to-parts) is through SolidWorks Explorer.
In this type of scenario, you will need to first locate and select the top-level file of concern (drawing or assembly) in the upper left File Explorer pane. Next, in the upper right Preview pane, select the References tab. The referenced files will be then displayed in the lower right pane. In the lower right pane, select the file(s) you want to establish a connection with using the Replace command.
These steps can also help to re-establish the relationship between files that may have been lost by perhaps inadvertantly renaming files using 'Windows / Rename' instead of the preferred 'SolidWorks Explorer / Rename' command.