If You have a chestplate enchanted with fire protection 1 and then try to take a protection 4 book, the game wont allow you to apply the book to overwrite fire protection with normal Protection.
MCPE-227835
Fire protection cannot be replaced by higher level protection enchantment
Resolved
Works As Intended
0
This is how enchantments have always worked in all versions of the game. It is not possible to overwrite an enchantment with an incompatible one by combining items. However, you can use a grindstone to remove enchantments from an item. You may want to read about enchanting on minecraft.wiki