Contact a good Realtor in your area and discuss your home's worth in relation to the current market. You will need to satisfy the mortgage lender by paying the loan off or otherwise, consider a "short sale" (paying the mortgage "short" of the amount owed).
For an excellent publication on the short issues, ask your Realtor for the State of Arizona's "Short Sale Advisory".
Which situation are you in, pay off or short sale?
Jeff Masich, Realtor
Arizona Homes and Land
The purchase of a second home will require a larger down payment if you are not planing on paying cash. You can also lease your exhisting home and call that income property and consider your next home purchase your primary residence. (check with your lender for details on income properties).
If you are considering purchasing a home, and obtaining financing, you do need to visit a lender first. The information you obtain from your lender will deffinately help you determine which real estate move is best for you.
Best wishes on your move.