>I want to change the printer direction.
>
>FIRST TIME...
>Command : SET PRINTER ON PROMPT
>Result : I can choose a printer (PROMPT works fine)
>
>SECOND TIME...
>Command : SET PRINTER OFF
>Command : SET PRINTER ON PROMPT
>Result : I cannot choose a printer (no PROMPT window appears)

I'm having the same result as you do with these commands, though the first time I tried my VFP hang and I guess I lost what I was working on at the moment.

Anyway, I didn't use these commands before. You may use sys(1037) instead, it seems to work every time.