Can cans of condensed milk be boiled until they become dulce de leche and then placed back in storage?

I'm already assuming the answer is no, but the price of dulce de leche insists I ask anyway. My grandmother used to make it by placing unopened cans of condensed milk in boiling water for a few hours. If I do this, could I do several cans at a time and place them back in the pantry? I don't want to pay $4.50 for a small can but I also don't have the time to boil individual cans every time I need it