I cant imagine they would take them back off you and refund you as the bolts have now been tightened and for all they know you have over tightened them etc, I certainly would be happy buying those as "new" now from the manufacturer.
you dont have to go stud conversion if you go for a 10mm say on the rear, you can just buy longer bolts.
I do. Wheel bolts poke through the alloy approx 18/19mm.
So I either need studs, and just slide the spacers over. Or shorter wheel bolts.