Danny,
The reason the instance is changing is because you are actually changing the symbol instead of the instance you want to change. When you change an instance, Flash doesn't change the symbol, but when you change the symbol (the original instance, the one from the library) it changes all instances. For example, if you had a symbol that was a blue circle and placed two instances on the board, then changed one of the instances to pink, you would have a blue circle and a pink circle. If you then changed the symbol in the library from a blue circle to a blue oval, your movie would then contain a blue oval and a pink oval, because any changes made to the symbol will affect the instances, whereas any changes made to the instances will not affect the symbol. Hope this helps, it may be easier if I were looking at your situation.