Here is the official press release put out in January
“those who have already got their hands on a 5G device and purchased one from Virgin Media from April 2020 onwards will automatically receive a boost to Virgin Media’s 5G services. Customers who take a 5G plan won’t need to lift a finger, their services will be upgraded with no need to swap their SIM or activate the service.”
Meanwhile new and existing mobile customers taking a 4G plan will remain on Virgin Media’s mobile service via EE’s network until VM’s agreement with BT Enterprise comes to an end in late 2021. Virgin Media anticipates that all of its mobile customers will have transitioned to the Vodafone network by early 2022.
So basically if you haven't bought a 5G phone from them from April, or signed up to a 5G contract you have to wait till you are moved off EE to Vodafone.
The only good news is that transition is going faster than forecast. You can request a new SIM but there is no guarantee it will move you to Vodafone. It may, or may not.
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