Long haul redemptions - book PE, change to J later?

[rtyuiop]

Hi folks,

Anyone with a good knowledge of award conditions know if it's possible to book a PE award (Oz-Europe, presumably on Virgin Atlantic) and then, if I end up with enough points before departure and there's suitable availability, change it to a J award (cancel and rebook presumably), without hitting massive fees/losing a heap of points?

Cheers,

Danny