Update 4: if you try to perform this on Xbox 360, IT WILL NOT WORK AND WILL RENDER YOUR WORLD AS "ONLY COMPATIBLE WITH MINECRAFT (2017)" FOR GOING ON ANY WORLD ON XBOX 360 WILL DO THAT. you're better of converting to Xbox one edition and THEN performing these steps.
Update 3: despite not wanting to, you will have to enable cheats to remove these items and to fly around the world. The world cannot be scaled by walking. Chunks (for some reason) seem to update better when in the air. Basically, it won't convert unless you fly.
And by native, I mean just converted from Xbox 360 and originally made on Xbox 360.
Update; oddly enchanted items (such as items that use enchantments meant for bows, weapons, other tools, etc. And items in your inventory) may prevent conversion from working. Update 2: just tested on two more worlds native to Xbox 360, they worked.
Chests from Xbox 360 in general. Xbox one chests may be fine but id do everything from both Xbox 360 and Xbox one just in case
The first few times I did this, I didn't get all chests. In this case, the world conversion may be higher than before (or not) but may still crash. If so, you probably didn't get everything. Chests from monster spawners (from Xbox 360) count too.
And I placed all old paintings into a chest
Oh and I placed the map in a chest so i wouldn't have to re-fill it. I kept nothing in my inventory.
BOYS, AFTER YEARS OF TRYING IVE FINALLY FOUND A FIX FOR OLD WORLDS NOT CONVERTING. The problem is simple but can be complex to solve. After years of getting the same and similar results when converting Xbox 360 world's to bedrock and Xbox one world's, I tried something. I thought, "maybe it's the old chest information and chunk info" and so, I got a new map on Xbox one edition, flew around until it was full ( this will require multiple for bigger world's), opened all chests at least once, and I rode all minecarts at least once. After saving, I went to bedrock on my Xbox one and converted, and what do you know? It worked. Finally. If only my og world wasn't deleted by the first conversion supporting build of bedrock. I converted the tu1 update tutorial btw in case you were wondering.
Is there any way I could get this posted on the official website support section where people can find this on the go? I feel not many people will read my solution