One way to switch between companies when you are already in a file is to use the File, Open Company menu option.  This brings you to the same Sage 50 - Open Company dialog that Open Existing does but does not require you to close the program and the company file in full, then start the program over again to get to the same place.

Once you are in that dialog, you have the options to change the path to the file.  You can use any of the places indicated by arrows in the image below to change the path to get to the correct filename.

I really like just clicking on the drop down icon to the right of File name (green arrow in the image below) as it lists all the most recently opened files of the same file type (*.sai).