If a logical unit is already open, an OPEN statement using the BLANK option does nothing but redefine that option.
As a nonstandard extension, if a logical unit is already open, an OPEN statement using the FORM option to switch between FORM='PRINT' and FORM='FORMATTED' does nothing but redefine that option. @
These forms of the OPEN statement need not include the file name, and must not include a file name if UNIT refers to standard input, output, or standard error.
If you connect a unit with OPEN and do not use the file name parameter, then you get the default file name, fort.nn, where nn is the unit number. Therefore, to redefine the standard output as a print file, use:
OPEN( UNIT=6, FORM='PRINT')