Sounds to me like you need +30/+30 as the potd weapon itself, and one already exchanged one (the ugly glowing none texture ones we got prior), to be able to exchange than your new +60/+60 to one of the ilvl 255 weapons at the end. Might be wrong but it sure looks more and more like that and knowing SE it's their new content, so they wanna push it a bit and make people run it more than once lol.
Might get the achievement however if you are planning on getting a place on the ranking system, you will have to redo these floors since that system wasn't available prior.
