move a file, replacing existing (if any), ideally by putting existing in the trash
I need to do some more research to get this right.  I feel like its close, but applescript still doesn't like it.  Maybe it will give you some ideas until I get back here to give it another shot.  Btw this is going to be so much easier when you are using the choose file dialog.

heres my non-working (yet) attempt.  If you don't be me to it ill try to finish the solution tomorrow. 

# 1. get desktop folder
set desktopFolder to (((path to desktop from user domain) as string)) as alias


# 2.set source file
tell application "Finder" to ¬
set theFile to ((path to file "1.text" of folder (desktopFolder)) as string) as   alias


# 3.set destination file
tell application "Finder" to ¬
set theDestination to ((path to file "2.text" of folder (desktopFolder)) as string) as alias



# 4.make it happen
tell application "Finder" to move theFile to theDestination with replacing