That was my question too. I got a 4G sim with my ultimate Oomph package back in February and I've seen that it never connects to 5G. So, for now this won't be upgraded automatically but it will be at a later date this year. Luckily I have a 5G plan too with EE.
Re: 5G on Ultimate oomph for existing customer (New contracts)
I've recently bought an iPhone 12 and phoned Virgin (789) tonight to ask about 5G. I was transferred to a helpful lady in the Oomph team who added 5G to my plan (after I confirmed it wouldn't change any other terms of my contract).
A few minutes later my phone lost signal and after a reboot I'm now connected to the Vodafone network. I also seem to be able to roam back onto the EE network (and the phone displays EE instead of Virgin). I assume this is probably just a temporary measure until everyone is moved over to Vodafone.
It might be worth a phone call if you're in the same position as me and have recently bought a 5G phone.