The short answer is that a QC cannot crack Bitcoin. One may be able to after Centuries, crack one address. Of course, that presumes that the coins don't move before then. If we ever have a computer that powerful, then cracking Bitcoin will be a very low priority task for such a device.
this thread isn't about quantum computers, there are other reasons that the collective may seek to change the algorithm of bitcoin or any other cryptocurrency.
this thread is about implications