Can the field data type created in ArcGIS Online be changed?

Note:
It is possible to change an integer field type to another field type using the ArcGIS REST APIs. However it has its own limitations and issues so it is not a recommended workflow. Only field properties can be modified. Refer to ArcGIS Rest APIs: Update Definition (Feature Layer) for more information.

No, it is not possible to change the data type of a field after the field is created in ArcGIS Online. Data types can only be selected during the time of the field creation. Alternatively, create a new field with the preferred data type and populate the existing field value in the new field using an Arcade expression. The workaround below provides the steps to add a new field, select the data type, and populate the field values in ArcGIS Online.

1. In ArcGIS Online, click the desired hosted feature layer. On the item page, click the Data tab.
2. In the Table view, create a new field. Refer to ArcGIS Online: Add a field for instructions. In this example, the Age_New field is created.
3. Click Menu on the header of the new field and select Calculate. In this example, Age_New is selected.
4. For Field, click the drop-down menu to select the field. In this example, Age_New is selected.
5. For Select language, select Arcade.
6. Click Next.
7. Specify the Arcade expression field as shown below. Replace <FIELD_NAME> with the desired field.

$feature.<FIELD_NAME>

8. Click Run to verify the expression, and click Run calculation to apply the query.

The values from the Age field are populated in the newly created Age_New field with the desired data type selected.