Switching to isomalt will drop ~50% in the caloric content, but change mouth feel and water retention a lot. A different confection will be the result.

Why not erythritol? Isomalt can cause some pretty severe gut issues (to which I can personally attest, unfortunately!). It's also still quite calorific (that is, it is metabolised) - erythritol isn't. I can eat it in high quantities to seemingly zero consequence. It's also got an entertaining mouthfeel, where it cools the mouth; so does isomalt, incidentally, but it's not nearly as noticeable.

I have done quite a bit of cooking with erythritol, simply swapping sugar out for it, and it usually works great. Erythritol is only about 70% as sweet, so it can sometimes be necessary to adjust for that, but really, most things are already overly sugary, so it's usually unnecessary.