Final answer:
When the currency field contains symbols or letters, it cannot be changed to a numeric type.
Step-by-step explanation:
One scenario when you cannot change the currency field type to numeric type is when the currency field contains characters other than numbers, such as symbols or letters. For example, if the currency field has the value '$100' or '€50', it cannot be changed to a numeric type because the symbols ('$' and '€') are not recognized as numbers. In this case.