Can I trade the iPhone 12 for an iPhone 12 Pro?

If you've owned your phone for less than 14 days, you can return the phone for full refund and buy the phone you want. As you can see from the link babowa provided, iPhone 12 is not an eligible device for trade in if you've owned it longer than 14 days.

Apple usually does not offer trade ins on a currently available device, but go ahead and try it here:

https://www.apple.com/shop/trade-in