I got my tongue pierced exactly 2 weeks ago, and I was wondering if it was okay to change the barbell I got pierced with to a smaller one. And if that is okay, how should I change it?

Changing it at 2 weeks can cause problems, such as infection or severe irritation. You should wait until 6 weeks post getting the piercing, that is when the piercing is stabalized enough to change the jewelry.

I would also like to recommend that when you do change it, change it to a plastic barbell. This lowers the risk of cracked, chipped and broken teeth that a metal barbell can cause.