90% coins trade at a significantly higher price per ounce than 92.5% sterling silver. Even though the sterling contains more silver, the rest of its composition is unpredictable, so refiners offer less for it. (There's also more demand for 90% coins as 90% coins, so often there's no smelting expense involved at all.) If the nominal price of copper goes a lot higher, I'd see the same thing happening with copper cents. I don't know how important "legal to melt" would be, if most of them are swapped without melting. They'll still be bulky, dirty, and unpleasant to deal with, though.