you won’t lose anything.
You can now activate a new sim if you loose one, or it is , damaged or you need a or nano sim, by doing a sim swap.
If your contacts are saved to your sim, back them up first.
1 Log out of gg
2 Click activate a sim
3 Enter the 6 digit code on the sim card.
4 Click continue
5 I the already a member section, enter your username and password.
6 Log in
7 Click yes I want to replace my sim
8 Click yes i'm sure.
Thats it. Your gg number, and account details will be transferred to your new sim.
I have all ready activated new SIM and put a goodybag on it,i have not lost any contacts,i just want my old Giffgaff number,when i try a SIM swap its saying i can not do this as i have reported it stolen and i havent
Your 'old' giffgaff number presumably belong to your 'old' account, this cannot be transferred to another account, nor to another active giffgaff SIM