Saving extras field to new field

I am hoping to update a deck to a newer version. This update will change all of my extra section that I have worked on. Is there a way to move all of my current extra section to a new field so that when I update, I have my previous version plus the new extra field? There are 40,000 cards so I am wondering if there is some Master setting to allow me to do this.

I believe the “special fields” add-on allows you to preserve certain fields when importing, but preserving your existing content will pulling in content from the deck may be tricky, as you won’t be able to update existing notes on import if the note type changes.