I would imagine they will be barred from trading in or selling the vehicle until it has the refit.
Of course the refit is going to degrade the vehicles performance so much that the resale value is going to plummet anyway.
So, basically current owners are screwed no matter what.
Current owners will likely get a buyback option or a sum that compensates them for any lost resale value or performance loss as part of the class action. It’s impossible to say how favorable those settlements will be to the owners right now, but it’s entirely possible they will make out on the deal.