Large amounts of data can potentially cause a timeout that can result in lost field customizations. Before running a field type conversion, you should verify the number of affected records included in the conversion.
Some ways you can verify the number of affected records include:
Review field usage. Depending on what the field is used for, you may know that it does or does not affect large numbers of records.
Create a saved search or workbook of the data with a condition such as the custom field value is not empty, or the custom checkbox field is not checked. The results can help you determine how many records would be affected by a type conversion. If a field is applied to more than one record type, such as Employee and Contact, you must complete two searches or queries and combine the totals.