Well, if you were to go into one of your ads now and change just one letter of the content, you will actually be removing the original ad and creating a new one in the process. This might not seem like a major issue on the surface, but you will in fact lose all the statistics for that previous ad which might ruin your split testing research and can even impact the quality scores within that ad group. This is actually one of the main reasons that Upgraded URLs (http://goo.gl/9F4Xg1) have recently been introduced as a feature (but that’s another story).
So if you can’t edit ads directly then you might be wondering how to go about this. The solution is simply to make a copy of the ad in question and then edit that new variation; I personally like to do this in AdWords Editor for the sake of speed, but the functionality is available to do the same in the live interface also. It might seem like a hassle to go through this process, but just remember that it’s for the benefit of the account and you might even find yourself using the older version of the ad at a later date anyway!