Adding variations to existing parent SKU?

I sell a product that comes in six different sizes, which are grouped under a parent SKU. I discovered two of the sizes were accidentally labelled with each other’s UPCs, and asked support if I could switch them. Ultimately, they said there would be no way to do this but to delete and re-add the listings. I’ve been trying to re-add them under the same parent SKU, but the threads I’m finding here seem to indicate that I CANNOT add new variations to the existing parent SKU, but rather I have to create a new parent SKU. Am I understanding this correctly? Do I need to delete the original parent SKU, and upload one new file with a new parent SKU, the four existing children SKUs that are unchanged, and the two new ones I’m re-adding?

Many thanks!

EDIT: I found where to add a variation by clicking “edit parent SKU,” but it wouldn’t let me relist my previous SKU even though it’s been deleted for well over 24 hours (about a week). I do still have this SKU listed in two other countries’ marketplaces on Amazon, so could that be causing this? Will I need to delist it from every country before I can relist it?