I was going to say cold turkey, but with the special needs you listed, that might not be the wisest choice. I'd leave it set up like you have it, then set a date for the Big Day. Maybe give her 3 or 4 days with the dual bed thing. Talk up Big Bed Day when you say "bye bye" to the crib. It could upset her to see the crib get disassembled or to see its parts in a pile, so you might arrange to say good bye, then take her somewhere while someone else gets the crib out of the house, or have someone take her out for a while so you can do it.
Speaking of Autism/PDD. Have you checked out the Neurosensory Centers of America? I have a couple friends who take their kids there (both with ASDs) and seem to like it and have seen results. http://www.neurosensorycenters.com/