Maybe not totally so just yet? I'm still able to use 'sell similar' from an older listing that had custom postage.

Scroll down to the postage section and click Add primary postage. The correct custom postage will show in the background, so just cancel the pop up without doing anything.

I can then go back up and make changes to title, description, price, etc. Pretty much anything except the category.

Admittedly this may not work if all your listings were opted in behind your back.