You might get a bit of chain rub against the chainstay by going from 6 to 7. You'll have to decide (if that occurs) whether you can live with that or not.

My bike is set at 128 mm in the rear so I changed out to a longer axle (one designed for 130 mm hub), placed 3 mm of spacers on the non-drive wheel side (and 1 mm on the freewheel side) and redished the wheel (less dish that way so it should be a stronger wheel). This may be more complicated than you want to deal with and someone here may have a simpler solution.