You will not need a new SIM. Virgin will have to move you off EE and on to Vodafone and that can be done seamlessly over the air.
Now here is the present state of play according to a post by Virgin on here
If customers upgraded to Ultimate Oomph after January 25th when 5G was launched they should already be on Vodafone and have access to 5G. If it was prior to that, then they will need to (re-contract) to a12 or 24 month SIM Only plan - or wait to be automatically migrated later this year.
Basically, pay more than you do now or wait
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