It was my understanding that you can never remove materia from an item once you've melded it - that's why it's called 'materia melding' or 'materia assimilation', it's not like FFVII where you simply clip in a piece of materia or can remove it again at will like batteries in an iPod. Of course though as I've never done materia melding (due to not bother with crafting), I have no experience with this so maybe someone far more knowledgeable on the subject can clarify this.