as long as selecting a new ‘Target’ hard drive works (i don’t personally use WD Backup, so i wouldn’t know for sure)
and if you’re copying the existing backup data off the 3TB to the 6TB … i dunno if it will keep adding new backup data to the existing data or overwrite the existing data ?
you might have to do a test … don’t move the data off your 3TB, copy it… in case
someone using WD Smartware had a bit of trouble though (link below) so … dunno, can’t offer any more advice unfortunately