Not sure if this helps or not... I have a world that I haven't been able to convert since the beginning. After reading this thread, I decided to experiment. I had 5 shulker boxes in my ender chest. I took them out and emptied the contents into regular chests. Everything stayed in the same chunk, didn't get rid of any contents, didn't get rid of the shulker boxes, I just emptied them and put them on the ground. It converted just fine the first try.
Not sure if this helps or not... I have a world that I haven't been able to convert since the beginning. After reading this thread, I decided to experiment. I had 5 shulker boxes in my ender chest. I took them out and emptied the contents into regular chests. Everything stayed in the same chunk, didn't get rid of any contents, didn't get rid of the shulker boxes, I just emptied them and put them on the ground. It converted just fine the first try.