A purchaser is the person on the retention document with "title" to the number plate - only the purchaser can add/change or remove nominees and complete the cherished transfer.
The purchaser is the person with legal entitlement to display a number plate- although you can never "own" a number plate- being the purchaser is the equivalent.
The purchaser can assign the number plate, add or change a nominee and extend the entitlement period.
It's not possible to add or change the purchaser. The purchase is the only person "entitled" to the number plate.