1. If you use untreated pine, I'm afraid you will be replacing them in a couple of years - even if you stain them. 2. This doesn't sound like a good idea to me. I would worry about the original decking rotting out under my new deck boards. If I were you, I would bite the bullet and do it right the first time.

Yes, it's pretty easy. Are the existing boards nailed or screwed down? If nails, pry the boards up; if screws, get a power drill and back them all out. Lay the new boards down, screw them in, maintaining a 1/8" gap, and you're good to go. There are some tips and tricks, but that's the gist of it. Composite boards often need closer joist spacing than wood (i.e., 12" on center instead of 16"). How far apart are your joists? If you post some photos, that might bring up other recommendations to share that apply to your specific deck.

We live on St.John in the Virgin Islands. Our average daily temperature is 84 degrees and never lower than 70 at night. It showers frequently and the deck outside of the short term rental apartment needs replaced. We added a treated wood deck to the rental apartment on the lower level of our home about five years ago. We are now having to replace deck boards and the surface was just painted with "Restore" by rustoleum to give it a few more months of life. We would like to use a non wood product to support and rebuild the frame then cover with Durarock as an underlayment to Travertine marble or a porcelain tile. Any advice appreciated. Thanks in advance. AlyceUSVI

Read the installation instruction for the PVC deck you decide to work with. From my experience PVC decking feel better on 12" center framing. We always tear out the frames and start with a new, flat, and properly constructed frame. Old frames take significant amounts of work and time to prepare for a quality installation. The frame is a critical aspect of having a good looking end product. Do it right, it will last a lifetime.