provided that you are actually upgrading your Pay As You Go account to a Pay Monthly then there is nothing to worry about.
Your mobile number along with any available credit balance you may have will automatically move to your new account
If by chance you've taken out a new separate "SIM only" then I'm afraid that the only way to retain your original PAYG number would be to port it out to another provider then back in again onto your new Pay Monthly SIM.
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