No, this is not possible. If you are eligible for lowering your deductible, you have the option to lower your deductible at the time of payment, after your rental request has been accepted.
The option to lower your deductible must be selected before the upfront payment. After payment is completed, it is no longer possible to change the booking and lower the deductible. If you do want to lower the deductible, you can only do this by cancelling the booking and making a new rental request. Please keep in mind a penalty might be applicable for cancellation.