According to a recent Head for Points info email, Virgin Atlantic are having to switch from Boeing 789's to different aircraft as a result of RR engine problems. I understand our flights to South Africa are planned to operate 789's. Also it appears according to the info email that the replacement aircraft will not provide Premium Economy seats. As we have already pre booked Premium Economy Seats as part of a Virgin Holiday package, what apart from a cancellation option, are the likely alternatives that will be offered to us?
I understand that the programme to do the necessary works to the engines of the 787s is due to be completed in the next few months.
As you have booked via V Holidays you have different options to someone who has booked direct with VS such as asking to be moved to an alternative carrier who also offers PE (though PE on one carrier may be totally different from that on another).
The other option (should it come to that) will be to accept the downgrade and the resulting refund of part of the fare you paid.
You should discuss your options with V Holidays but you don't have to accept their first offer of an alternative.