Easiest run them through the table saw with a carbide blade, little WD40 on the blade before each cut. Nip the cut side corners on a 45 then take a grinder with 40 grit sanding disc and round them or use a jigsaw.
If it was me I would pop the brackets off the back cut 4" off the one side, end up with 12" and 4" pallets.
Then put the brackets back on the 12" and buy brackets for your new sleeve pallets later.