Using KiXtart 2010 4.50
Code:

Break On
Move "D:\MyFolder\MyFile1.txt" "\\MyComputer\MyShare"
? @ERROR ": " @SERROR
Copy "D:\MyFolder\MyFile2.txt" "\\MyComputer\MyShare"
? @ERROR ": " @SERROR
Move "D:\MyFolder\MyFile2.txt" "\\MyComputer\MyShare"
? @ERROR ": " @SERROR

The kix2010 manual tells for the "Move" command that:
Quote:


MOVE overwrites existing files without warning.



But when moving to an UNC style path and when the destination file exists, the file is not overwritten. The Error Code is 80, telling it exists.