Roger, as long as the plywood that is down is sound, then you can go right over top. Engineered floors, because they are floating, are actually quite flexible. As long as you don't have more then 1/2" deflection in 4' you should be fine. It is pretty much like using a laminate. Click
HERE to see another thread about flooring where I had commented on engineered wood flooring. I plan on doing cork in my trailer when I get it.