Just like writing a story, your roleplaying world may need to be revised from time to time. As you play in your world you notice things, places, people which need to be changed in one way or another. Perhaps the ability is not balanced, perhaps you found a better way to do something. Here are a few ideas I have for putting these new items into your world without messing it up.
1) Keep track of your changes. Sometimes, in our busy lives, we forget what we've changed and players like continuity. I changed my "Priest" title from "Mage" to "Priest" because it was just too confusing as to who was a Mage versus who was a Priest. I had to go back through all my notes, (and stories) and fix all my "Mages" who needed to be "Priests".
2) Try not to make huge changes. Don't suddenly make your world magical after having limited magic. The repercussions in your campaign would be huge. Try to keep them small, such as an item's ability changing from "protection from magic" to "protection from fire". Try to keep the changes to your world small, unless there is a very good reason to change.
3) If you have to make a huge change, justify it. I'm not sure about you, but when Dragonlance changed from super magical to no magic, I protested with my wallet. Same can be said about the Star Wars series that went to a Borg like race. Huge changes in the way a world has to make sense and not change the feel of the world. Now if a huge earthquake rips the world apart, but doesn't change the use of magic, (or lack of use), but adds to the tension, then feel free to use it, just do so with caution.