Open a 0.8.x world in 0.9.0, then close it.
Make a new 0.9.0 world, doesn't matter if its survival or creative, close it.
Open iExplorer, go to Minecraft PE > Documents > games > com.mojang > minecraftWorlds
Go into the files for the 0.8.x world, copy db.
Then go into the files for the 0.9 world, paste db.
Done. Your 0.8.x world is infinite.
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Make a new 0.9.0 world, doesn't matter if its survival or creative, close it.
Open iExplorer, go to Minecraft PE > Documents > games > com.mojang > minecraftWorlds
Go into the files for the 0.8.x world, copy db.
Then go into the files for the 0.9 world, paste db.
Done. Your 0.8.x world is infinite.