No, this cannot be changed. The dropdown option of designating an early payment discount on credit notes is part of a hard-coded feature. You will notice there are originally two choices: Early payment discount and Custom. When you choose Early payment discount, the free text field disappears. The credit note is then part of a larger process for handling early payment discounts. It is almost as though it is a different type of transaction entirely.
If Custom is chosen, the free text field remains and functions like the normal
Description field on a debit note or other transactions. The text you enter there will appear in the Credit Notes tab list the same way an ordinary description appears in the Debit Notes tab list.
You are not really meant to make this selection yourself. The selection occurs automatically when early payment discounts are processed according to instructions in this Guide: https://www.manager.io/guides/14497.