Change prepayment invoice status from Permanent to Temporary

Dear All,

We are using Oracle EBS R12.1.2; DB: 11.1.0.7; we need to do the following:

1. Change status of Prepayment invoice from Permanent to Temporary; this invoice accounted status is Yes; and all fields are disabled.

2. Since we don't have any business needs to use permanent prepayments and all our prepayments should be applied to invoices; What is the root cause? and how to avoid that in future?

Thanks in advance;

Shareef