You already have the right mattress for your weights. The plush version isn't as strong or strong enough to hold up over time.
Be it Luxury Firm or Plush, the memory foam upholstery will begin to sag within a matter of months.
You will need a latex topper to extend the life of the mattress...regardless of which one you choose.
My suggestion is to keep what you have, as it is the better of the two.
Take the exchange fee and put it toward the EverEden 2" Soft Vytex Dunlop or Talalay Latex Topper.
This will work out for you, much better, over the coming years....and, extend the life of the mattress well beyond what the manufacturer intended.
Here is a link to those latex toppers.
Soften the surface of a too firm mattress. Insulate yourself from the memory foam in your mattress to Sleep Cool. Extend the comfort life of your mattress.
Delivery is free and there's no sales tax.