iPhone upgrade program question

I have a bit of confusion on this. To start the upgrade program do I have to trade in my current iPhone that I paid in full for previously? If so this somewhat seems like stealing. Someone that didn't have a cell phone that started the program wouldn't have to trade in a phone to start? Or someone with an Android phone wouldn't have to trade in to start? So if I understand this correctly, if you currently have an iPhone (even if it was paid for in full) if you want to start this upgrade program you have to give them that phone? I just want to make sure I understand this correctly. That seems all sorts of messed up for the reasons I stated above. It's essentially a different cost to start depending on your current phone situation.

Thanks for your answers ahead of time!

 

Thank you. I knew that didn't sound right. Apple needs to reword their information about the program because their website says "To get started just trade in your current phone".