HiDavid,
Thanks for posting on our forum
As your concern you want to re-write the file when Move or copy items from a SharePoint document library,to another SharePoint online documents library, then you can see two option Keep both and Replace ,we you click Replace it should be re-write to exiting file in other document library .
Meanwhile, we would like to confirm: Did you see Replace option?
Let us know if you need additional assistance.
Best Regards,
Waqas Muhammad