Could anyone clarify the specifics about the Apple iPhone Upgrade Program?

Hello experts of the Apple community.

I am new to the overall Apple experience, and I was wondering if anyone could answer my question.

First off, I am in the middle of deciding whether I should buy an iPhone 11 now and upgrade to an iPhone 12, or just wait for it without buying an 11. But the thing is, that my phone is quite old and I'd like a new one asap. So, this is where the question comes in. I recognize there's this program called 'Apple iPhone Upgrade Program', and I was confused about how it works specifically. In my case, could I monthly pay an iPhone 11 until the release of the new iPhone 12 and change my plan? I just don't know if Apple allows customers to switch in between contracts. I realize that if it is possible, there may be extra fees but I just want to know if it is actually possible excluding the pricing issue. Would I have to wait an entire year until this program offers me a new phone, or could I use the iPhone 11 for approximately five months until the new iPhone 12 comes out? I don't know if my questions are clear, but in a nutshell, I just wish to know if it would be safe to buy (start monthly payment) the iPhone 11 with the upgrade program and switch to the new 12 after its release. Just wondering if the program lets me stop after five months to switch to a new phone. I'm really torn about which new phone to buy myself after a long four years of suffering, and I hope someone could help me please!