Risky but fast answer if invoices aren't created from Sales Orders: Steps: 1) run saved search that includes all lines and download. Include line IDs and ANY fields that your company fills out, even if you are not wanting to change them. 2) remove unwanted lines from that saved search results (note keep a backup of the first copy in case you screw something up) 3) run an import with "overwrite sublists" on. 4) run search and validate. Test this on a small sample - or preferably in sandbox. Note* this goes out the window if invoices were created from Sales Orders. I would probably ask people to suck it up and update manually.

Cool, I'll try it on a handful

It's "override sublist" and it's on the CSV import under advanced options

Got it