Overbooked flight—asked to volunteer for cabin downgrade

I recently received an email from Virgin Atlantic that my upcoming LHR to JFK ‘flight is overbooked in our Upper class cabin due to recent changes in our flying programme. As a result we’re unable to accommodate all booked passengers on this service.

‘To prevent disappointment at the airport we are looking for volunteers who would be prepared to accept a downgrade to our Premium cabin.

‘If you are able to help us out by accepting this change we will be pleased to offer you a choice of one compensation package per person from the below options for the inconvenience this will cause.’

The compensation options are:
– a voucher exchangeable for a free round-trip ticket, inclusive of tax, in Upper Class, subject to reward seat availability and valid for 3 years;
– 100,000 Flying club points.

I wondered if a change in aircraft is the case, but since there is only one less Upper Class seat in the A330-300 & B787-900 than in the A330-900neo, and the A350-1000 has 12 more Upper Class seats, I’m unsure.

There are two people in my party, both of us being Red members, and the aircraft is an A330-900neo. The original booking was cash payment for Premium Economy last summer, then I used points to upgrade to Upper Class (paying taxes, fees & surcharges in cash) shortly afterwards.

Based on the above, does anyone have an idea how likely it would be for my travel party to be bumped down or otherwise not be able to take the flight if not enough people volunteer for the downgrade to Premium?