Can you put an 18 gauge into a normal ear piercing?

I want to get 18 gauge circular barbells, but I have just normal ear piercings that were done with a gun. I measured the earrings I got my ears pieced with and they are as big as an 18 gauge, so I was wondering if I could stick an 18 gauge in them my ear piercings. If I can't, do what do I do? And if I get another normal ear piecing with a gun and I want to change it to an 18 gauge, how long should I wait before stretching it?