Probably not till after they sign a new contract. The present one has a couple of years to run.
Also remember that with Virgin moving to their own core network changing signal providers is not going to be the hassle it was in the past as there will be no need to issue everyone with new SIM cards.
Time will tell.
It's pretty rare for MVNOs to change their host networks, but you're absolutely right in that it can happen - I seem to recall when Lebara (think it was them) moved between Orange & O2 many years ago it caused all manner of issues - I've a feeling Asda have also done it from VF to EE. Another reason to "be aware" with MVNOs.
I have a suspicion that V-Mobile having their own core is for other motivations than facilities such as VoWiFi - I suspect it's more for fixed/mobile integration & comvergence, however that's pure speculation and could be balderdash!