Hi germannubie,
on NI´s site is only an instrument driver for LV6 available.
Keithley provide an
instrument driver only for LV6, too.
There is one possibility to open each vi with LV 6 listed in the llb and save each vi with options for previous version LV5.1.
But think of one: there is no gurantee given for downwards compatibility.
Regards
Indiana