I have just bought myself a dji mavic pro & I'm going to use it to replace my phantom 3 standard. I have 2 questions.
1- Do I need to reset my dji account & adjust it or can I just plug in & go ?.
2- What lead do I need to connect the remote to my iPhone?.
Thank you in advance for any help.

You do not need to reset your DJI account. However, you'll need to install DJI GO 4 on your mobile device. Both DJI GO (3) and DJI GO 4 can be installed on your mobile device at the same time.

The Mavic comes with a short lead that can be connected between the iPhone and Micro USB port on the left side of the remote controller. After disconnecting that lead, you can also connect the iPhone to the USB port on the bottom of the remote controller using any Lightning cable.