Credits are only able to be used once. Once a credit is utilized in a projection, it cannot be restored.
The Traffic Projection Tool needs to operate on a credit system because of how our costs work internally. When you setup a new projection, our tool goes to work building your keyword database. When the tool builds your database, it uses multiple API calls to aggregate thousands of keywords.
The Traffic Projection Tool covers the costs of the API calls for you. Once you build your database, the costs are set and we cannot go back.
However, you can edit projections once setup.