If i have a protection 3 piece of diamond leggings , and i try to enchant them with a protection 1 book in an anvil , it eats up my xp but doesn't enchant them at all , and if you try to do /enchant @p protection 1 it says "diamond leggings cannot support this enchantment" , but it works if you have no enchants on them and then type /enchant @p protection 1
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I'm able to confirm the first part of your ticket though and extracted it to MC-174782.
As Protection I is a lower level than Protection III, it won't combine to a higher level enchant.