That doesn't mean you shouldn't do it. Eliminating the forward mortgage eliminates the monthly payment on that mortgage, which frees up your income for other purposes. Further, the unused part of the line ($5,000 in my example) will grow by 4 percent to 5 percent a year, depending on future interest rates, so you will have more to draw on in the future if you don't use it up now.

Obviously your game plan won't work unless the down payment on your next house is 50 percent or more. You can draw only $105,000 on a $200,000 house because you are still very young and the reverse mortgage lender is going to have to wait a long time before he gets his money back. If you were both 74, you could draw over $120,000, and would only need to put 40 percent down. And if you were both 84, you could draw over $140,000 and would only need to put 30 percent down.

Since you must put 50 percent down anyway, you should consider putting 100 percent down – paying all cash for your new house. If you have the cash sitting in low-yield investments, that would be a prudent move. You would then have access to your entire HECM credit line, which could be drawn on as needed, while the unused portion would grow by 4 percent to 5 percent a year.

If you can't make the purchase without taking a forward mortgage, make sure the mortgage meets your needs, which are atypical. It should not matter much to you whether the interest rate is adjustable or fixed, or whether the term is 15-years or 30 because you are only going to have it a short time. What does matter is the upfront mortgage costs, which you want to minimize.

Upfront costs consist of points, which are a lender charge expressed as a percent of the loan; lender fees expressed in dollars covering different services, but only the total amount matters; and mortgage broker fees, which can be expressed in percent, in dollars or both. The trick to avoiding all these costs is to pay an interest rate high enough that the lender will pay you points, called a "rebate." The best instrument for this purpose is a 30-year fixed-rate mortgage (FRM) because they carry the largest rebates.

For example, a lender offering a 30-year FRM at 6 percent and zero points might quote 2.75 points at 5.5 percent, and rebates of 2.125 points at 6.5 percent or 3 points at 6.75 percent. You want a rebate large enough to cover the other lender fees plus the mortgage broker's fee. If the rebate is larger than needed for that purpose, however, it won't go to waste; the excess can be applied to third-party charges, such as title insurance.

Also, you must make absolutely sure that your loan does not carry a prepayment penalty. Pay careful attention to the Truth in Lending disclosure statement that you receive. Near the bottom, it will say that if you pay off your loan early, you either may, or will not, have to pay a penalty. If it is marked "may," it means you will have to pay a penalty.