EE sending me a new SIM card because I opted for a new deal.

Sounds daft but I thought it was just a matter of them changing things. It’s only slightly different from the one I’m on. It’s supposed to be coming tomorrow. Beside worrying how I change it I am being asked to register it when I get it.Do I do that before or after changing it over. I am so worried I might be without a phone. Also will my contacts all be on the new Sim? I remember when there was a change from pay as you go to my contract the contacts were missing. The other thing is that you used to be able to phone EE on 150. Now they direct you to a link and when I looked it was confusing. They call it interactive. No person so suppose AI. I wish I had just continued with my old contact at higher price. I know I sound idiotic but my 85 years are catching up on me. Can anyone help this confused old woman?