Did you seek advice before challenging the existing award? Did you ask them to look at both components or just the one?
It would also be helpful to understand something of what the change was although I appreciate you may not want to post that on here.
Okay, well as has already been said you need to start the MR process. It crossed my mind that the sudden use of an aid may have naturally dropped the points but looking at the descriptors I can't see how that would have worked in your case so it really comes down to how much detail you wrote. They may have assumed that the use of an aid increases the distance you can walk and, following on from that, have deduced that you must therefore be able to do more on daily living. None of that necessarily adds up if the evidence is addressed properly so all you can do is address any assumptions on their part head on in the MR.
It is worth reviewing what you wrote when you asked for the change of circs. A lack of detail is often what messes it up.