Unless the application / command line program (which you are using to send the contents of a nominated file to a target printer) insists upon a particular extension, you don't even have to rename the file as Dennis suggests, although it makes sense to do so, in order to easily see which file is which.
If you use the PRN File Print tool in the PCL Paraphernalia application (available via http://www.pclparaphernalia.eu ) to send the contents of a nominated file to a target TCP/IP-connected printer, the tool will (by default) show a list of files with extensions of .prn, .pcl and .dia in the selected directory.
And, just to avoid any misunderstanding, you can't turn the content/type of a file to a different content/type merely by changing the file extension; renaming a file just does that - it doesn't change the contents at all.