In ArcGIS Pro, attempts to alter the field length for a field in an attribute table fail, and return the following error message,
Error: ERROR 001624: Failed to alter field length
A smaller value is entered to decrease the field length. This is a known limitation. In ArcGIS Pro, while the field length of a field can be increased to include larger values, it is not possible to reduce the field length by specifying a value smaller than the existing value. For example, if the existing field length is 255, the error message is returned when attempting to alter the field length to 250.
Create a new field with a smaller length field value and repopulate the new field. Refer to How To: Decrease the character length for a text field in ArcGIS Pro for instructions.