Once a Gift has been created and saved, the Gift Type field is locked and cannot be changed.
There are a number of conditional fields on Gifts that are dependent on the Gift Type, including Credit Card Type, Check Number, Noncash Type, and Stock Ticker, just to name a few. Making changes to the Gift Type can cause conflict with data in these other fields.
In order to change the Gift Type on a Gift, you will need to delete the Gift and then re-add the Gift with the correct Type.