An app builder needs to change the data type of some custom fields.

Changing the data type of custom fields in Salesforce has specific limitations that need to be considered to ensure system integrity and prevent errors:

A. It is not possible to change the data type of a formula field to any data type. Formula fields are calculated based on other field values and cannot be converted into a storage data type because they do not store data themselves.

D. It is not possible to change the data type of a Text Area (Long) field to Text. Text Area (Long) fields support up to 131,072 characters, which far exceeds the 255 character limit of standard Text fields. Converting such a field to a smaller capacity field would potentially lead to data truncation or loss.

For official guidance on data type changes, refer to Salesforce's Custom Field Considerations.