Here's how to change the field type of a Custom Field.
1. Go to Admin and click Field Management.
2. Find and select the desired field.
3. In the Field Actions drop-down, click Change Type.
4. Select the new type.
Score and formula fields cannot be changed.
5. Read the warning, then click Change to confirm.
The warning message you see will vary depending on what field type you're changing from/to.