you need to call the Virgin mobile team on 789 from your new Virgin SIM.
Tell them what you want to do, give them your mobile number & the PAC code you obtained from giffgaff, they'll sort it out for you.
If you want to say thanks > click 'Kudos'.If this has helped answer your question > click 'Helpful Answer' also.
I just spotted this & thought of you
I don't know whether it's still a current promotion but it is on the Virgin web site.
Definitely worth a mention when you give the team a call.