Let's say copper is 2.5 cents per penny and they change the law and you can now melt them down. What makes you think that you will receive 2.5 cents per penny? People that have the ability to melt these down will be buying them at about 1.01 cents per penny, as they will pay wholesale. Even if you melt them down in your back yard, you still have to sell them to someone, and you will get a wholesale price. Factor in the price of gasoline, containers and your time.
Also, what makes you think that the gov't will change the law to allow you to melt these down when the gov't can melt them down themselves and make that money instead of you? They could simply demand that you return pennies to the gov't or you can bury them in your back yard, much like what they did with gold in 1933, but either way, you will not make a reasonable profit off of pennies no matter what you do.
In addition, the fact that you are hoarding pennies somewhere means that the money is not in the bank earning interest. Every year, you lose about .75% (approx. the maximum average interest rate for a money market fund) plus an additional 2.3% (approx. the average inflation over the last few years). So, every year that you keep pennies, you lose over 3% on the value of those pennies each year, assuming the price of copper stays roughly the same (it has been about the same for the last 7 years). So, over the last 7 years, hoarded pennies have returned a loss of about 15%, while putting that same money in the bank would have resulted in a gain of about 6% (with compounded interest), just by putting it in the bank. That's a 21% difference. Think about getting that same money and investing in bonds, mutual funds, or even gold/silver over the last 7 years. Your returns would have been much higher as well as being indexed for inflation.
The bottom line is that you will not make any money by hoarding copper pennies. Spend them.