Universal Containers has a new custom object for Invoices that includes an Invoice Number field. Before the Invoice object can be used, invoices will be migrated from an external system maintaining their current Invoice Number. After the migration, each new Invoice created in Salesforce must have a unique Invoice Number.
How should the app builder configure the Invoice Number field?
To address the requirement of maintaining the current Invoice Numbers during migration and ensuring each new Invoice has a unique Invoice Number generated by Salesforce afterwards, the best approach is to use two fields: a Text field for the original Invoice Numbers and an AutoNumber field for the new Invoice Numbers. This strategy preserves the existing data and meets the uniqueness requirement for new entries.
Answer is A https://developer.salesforce.com/forums/?id=906F0000000DCbOIAW 'If you convert an auto-number field into a text field, the data in that field remains unchanged. Also, you can safely convert a text custom field into an auto-number field without losing your data. Converting an auto-number field into any other data type results in data loss. Auto-number fields can contain a maximum of 30 characters. Before converting a text custom field into an auto-number field, change any records that contain more than 30 characters in that field.'
C. Create a Text field and mark it as a unique external ID field. All other answers don't make sense.
In the case of answer A, I think that changing from text type to automatic numbering type will cause data loss
A "Also, you can safely convert a text custom field into an auto-number field without losing your data."
A. Create a Text field, then change it to AutoNumber after the migration. This option allows the app builder to first migrate the existing Invoice Numbers from the external system, which would be stored in a text field, and then change the field type to an AutoNumber after the migration. This ensures that the existing Invoice Numbers are preserved, and that each new Invoice created in Salesforce will have a unique Invoice Number generated by the AutoNumber field.
It should be A, coz it clearly states in the question 'After the migration, each new Invoice created in Salesforce must have a unique Invoice Number.' whereas unique external id won't populate automatically and mainly used for matching.