What happens if I trade my iPhone 6 for an iPhone 6 PLUS?

I want to trade my iPhone 6 (used) for a 6 PLUS (used) and I want to know if I have the 6 plus will it allow me to transfer everything from my old phone to the new one (Used of course) and everything would be perfectly fine? Please reply

When you sync with iTunes, everything will port over to the new phone

If you backed up your data