Yes, you can do that.
I want to change my number but keep the sim. Is that possible? I'm going to stay on giffgaff though. Is there a way to do this or I have to get a new sim? Any help would be nice thank you.
either activate a brand new sim from within your account, or ask an agent to change the number for you
either way, you will have to top your account up by at least £10 - £ 0 to activate a new sim, or £10 to activate the new number if you ask the agent to change the number for you
hope this helps