When can i change my tongue piercing to a smaller barbell ?

I got my tongue peirced 5 days ago and I want to know when can I change the barbell to a smaller one .. my tongue didn't actually swell as much as piercer said .. I take ibprofin before I go to sleep and sometimes when I wake up. my tongue still is a little sore but nothing I can't bare.
oh and what happens if the piercer puts a small barbell in when piercing your tongue by accident and not the longer barbell???