If the cartomizer is full, you'll need to get the juice out before you can add more. They only hold about 1 milliliter of juice on average, so a full one has that much in it. You'll need to either vape them until empty or flush them out and dry them before you can add more juice, especially if you are trying to get rid of the tobacco flavoring. You can use hot water or grain alcohol like Everclear to flush out the original juice. Some people say a turkey baster will fit on some cartomizers, and you can use that to draw the liquid in and out to flush out the juice. Or an infant's liquid medicine syringe also works when you fit the rubber cone over the cartomizer.
Once you've flushed them out, you'll need to dry them inside. Use the syringe or baster or whatever to blast as much air as possible through them, then put them out in the sun, in front of a fan, or in a warm spot. I've heard putting the oven on it's lowest setting,then turning it off when it's warm, then put the cartos on a cookie sheet or something for awhile, making sure it's not hot enough to melt the cartos!!