What type of boarding that needs supporting if lifted ?.
If tongue and grooved you will have to pull up the edge board first which if you can't slide it sideways away from groove/tongue you will have to cut down the joint but test for electric cables or pipes that may be fixed to sides of the rafter.
Lift all the panels sliding each one away out of the groove or off the tongue of the next one number them so the go back the same place. Lay insulation keeping eves clear for venting roof then relay the boards.
Warning do not cover downlights that clip into the ceiling with insulation leave at least 4 to 6 inches all around the light clear or the lights will over heat and may cause a fire. Seek further advice on lights though.
BB
January 2010