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.