Here's my suggestion:
Today or tomorrow, while you can still find them in the stores, pick up a pair of shorts... denim if you choose.
They're your just-in-case-WDW-feels-hotter-than-the-face-of-the-sun backup. If it turns out that you don't need them, donate them to charity.... I'm guessing that returns will be an issue. If you do need them, you can probably get more than one wearing from a pair of denim shorts before you wash them, and the resorts all have washers and dryers.... you can throw them in, go for a swim while they wash and dry, and be good to go.
As much as I love sun and heat, I wouldn't consider wearing jeans in WDW. Then again, we go in August.