Better late than never...
If you have 1.5 inch bars, you will need to add 1.25 inch bars for them to build brood comb on. I'm sure they've started the nest on the oversized bars, but they will move honey and stores all over the hive. My guess would be a cutout.
Making comb is expensive to the bees, and they wont like to make more unless they need it. You won't be able to just remove the bars without removing the brood, so the quickest/easiest solution I can come up with is to move the brood over to the proper sized bars with a cutout.
They will build the nest on those size bars, but will want to set the comb size to what they like, and will probably build the nest on 2 bars at the same time. If it's honey, they'll generally extend the length of the cells to take up the extra space.