Best answer by Mohammed
Welcome to the iD Community.
You can change a plan at any time, be it the start of the month, middle or end.
There are a few restrictions though, the main ones being:
You need to be on a plan for at least 31-days before changing to a new plan
You need to have at least been with us for 30-days before requesting your first plan change
You need to ensure you have no outstanding balance/overdue balance.
You can carry out a tariff change from within the iD app/MyAccount.