So, in the relatively-far future, humans, in their endless quest for immortality, develop a form of mind uploading. This involves scanning the brain, cell by cell, and creating an electronic model of the neurone patterns in which all the information about the subject, their memories, likes and dislikes, is housed. This information represents the living soul.
However, in order to have the “soul” transferred rather than simply copied, the process has to destroy the nerve cells as they are scanned, so that the information is removed from the brain as it is reconstituted in digital form.
Why would the scanning process destroy the neurones? I am leaning towards a reason involving radiation or something like that destroying living tissue, but am open to suggestions. Please note that a reason need not be particularly in-depth, just believable.