as you are billed in advance for your tariff this is to be expected.
As you are probably aware, you have the right to cancel within the first 14 days without penalty. Only being charged for any allowance used.
Have you received and activated your new SIM yet?
If not, any monies taken will be refunded.
For peace of mind, I would call customer services again (789 / 0345 6000 789) just to clarify your intention.
When you say "upgrade your service". Were you previously "Pay as You Go" with Virgin Mobile?
If so, were you transferring your mobile number and credit to the new "Pay Monthly" SIM?
If you haven't activated it yet, not to worry.
If you have. You will also need to discuss returning to PAYG and prefered tariff.
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