Not at all guaranteed - similarly to how cancelling a cheap ticket does not mean that there is suddenly 1 seat in the cheap fare bucket, instead there is now 1 in the expensive fare bucket and a revenue manager rubbing his hands in glee (metaphorically).

Don't do it.

However, if you do book the CX ticket with BA miles but would prefer to use AA miles even at the lesser value, you could watch for close-in CX availability and if it arises, book with AA miles then cancel the BA. That is, obviously, safe.