Blocked custom field type changes
Key Takeaway
Some custom field data type changes are blocked outright. Most blocks come from one of three causes: the field's own type, what depends on the field, or the field's data and relationships.
Salesforce lets you change the data type of many custom fields after you create them, but this option isn't available for every data type. You need to know the blocked changes so that you can choose the right type up front and spot why a requested change isn't possible.
Blocks from the field's own type
Some fields can't be converted because of their own type:
- Formula fields are read-only and can't be converted to any other data type. The reverse is also blocked: no other field type can be converted into a formula field.
- Custom metadata type fields can't change type at all after they're defined. For example, a Text field can't be changed to Text Area.
- In Salesforce Knowledge article types, a file field can't be converted into any other data type.
Text Area (Rich) fields have only one allowed conversion: to Text Area (Long). After this conversion, images are deleted the next time someone saves the field. Markup is hidden, but it stays on the record until that save, so you can still restore it before saving.