Overwrite all .extension in specified directory

Hi Guys,

I need a little help.

I need a script which overwrite all .xyz (example) files in a specified directory.

Example:

C:/A/1.xyz
C:/A/2.xyz
C:/A/3.xyz

I have to overwrite them with 4.xyz but must keep the original file name.
I don't mean EXE files I mean specified extension for an app.