Ironically, the "normal" upgrade to a Jr Suite only has 1 bed, so that won't work for you. The penthouse suites have a proper king and sofabed and are actually pretty good value. You are getting twice the space for just ~$100 upcharge. You might want to bite the bullet and go that route.

As Gold, the "destination fee" they charge will be waived, and you can ask for late checkout.