Second hand phone

[abstractzebra]

Can anyone help as I've never done this before.
I have an iPhone 6 which is on a sim only contract.
As it is getting out of date, I'd like to buy an unlocked phone from somewhere like CEX and give mine to them for a small amount of money off the new phone.
When I put my SIM card into the new iPhone, will all my stuff from my old phone transfer after I've logged in?
Also, will it be safe to hand over my old phone minus SIM card? Will there be any of my information on it? I can't clear it as I've no guarantee that there will be a phone I want to buy and I use mine all day every day!
I've only ever done this before at a phone shop and when I've used my new phone everything has transferred over.

You need to make sure your phone is backed up to iCloud and then that means when you set up a new iPhone all your details and apps etc will transfer over. It's nothing to do with your sim, it's iCloud that will transfer it. It's really easy though, when you sign into the new phone with your apple account details, it'll ask if you want to restore backup.

You will need to delete everything off your current phone, nowhere will take it whilst it's still linked to you as it's no use to anyone. But this doesn't take long so can't see it being a problem to just erase it when/if you decide to trade it in. Or if you erase it beforehand but don't find another phone you want to buy, you just put your backup back on your current phone.