Hi RapidAmp,
Thanks for your post and welcome to the forums. It's great having you on board with us in the Community.
Where you are an existing Virgin Media customer, but not yet an O2 customer, you would need to look at changing to a Volt package. To do this speak with an agent who can go through options with you. It is worth noting that whichever speed you choose for your Volt package will be the speed you get as it's already boosted so you wouldn't get another boost once your new O2 SIM is activated.
If you wish to go up to 1GB, and you don't have a Hub 4 then one will be provided as part of your upgrade however there may be activations charges for this. You can check this with the agent at the time of upgrading. Once you've chosen your Volt package, an O2 SIM will be ordered and sent to you. This will also already include the boosted data.
Hope that helps answer any queries you may have.
Thanks.