A soft credit on a constituent is a link to the "hard credit" gift record from the actual donor. That means its values, such as gift type, come from that main  "hard credit" gift record. The only fields are unique to the soft credit that can be edited are the soft credit recipient and the soft credit amount.

To change the gift type of a soft credit, the "hard credit" gift record must be edited: How to change the gift type on a gift