Updating file with new extension

Hello,

We are setting up a vault, and have a case that will cause some issues in PDM with versioning and Revisions.  We have a bunch of old Excel Sheets (.xls), and when they get updated, they are saved in the new .xlsx format and put into PDM.  Most of the work from xls revsion to new file is done outside of PDM.  When a use drops the new file "on top" of the existing, checked out version, it puts it in as a new file.  Any recommendations to make this work? 

SolidworksSolidworks Pdm/enterprise Pdm