Multiline Set Keys Form
Export the keys to a csv then search and filter if needed, delete any that aren't being changed and then re-import.
That's exactly how I do it.
When you export this form it actually comes in two parts.
1- it will list all the extension.
2- it will list all the keys for each of the extensions.
I generally export it, delete the top section where all the extension are listed, insert a new page, copy all the header information (top 3 lines) into it. Next you can sort based on the key directory assignment and copy the keys you need to change out into the second sheet. Change all the keys to "not assigned". You can keep the label if you want or change it. Save it as a CSV and upload it. This has not blanked those keys.
Next change the keys on the spreadsheet to what you need them to be, save it and re-upload it.
It actually goes a lot faster than it would at first appear. After you've done it a couple of times it will really go fast.
I've done this with Excel and Libre Office.
Ralph