Hi, Roxanne.
Welcome to the Three Community.
The team will often send out a Pay as You Go SIM along with upgrades in case you plan to keep your old phone. You can pop the SIM into it and give it to a family member, or just use it as a backup when needed.
The new Pay Monthly SIM should be a replacement for your existing SIM. It should activate on it's own once inserted into your phone. Your number should also remain the same as it was before the upgrade.
Pete.
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