Yes. Just call 789 to speak to them and negotiate a new contract if eligible. Doing an upgrade will allow you to keep your same SIM and phone number.
If not in contract, you can also consider the o2 deals as well. Especially if you have Virgin Media services at home too, the Volt benefits they offer may make it an even better deal than Virgin Mobile... They can give you a PAC code to transfer your number to o2 if you wish to do this.
Even if you don't move now, as an existing Virgin Mobile customer you will be migrated over to the o2 brand in due course following the VM-o2 merger, so you'd just be speeding up the inevitable.
