You can't assign one field to another using sql statement. To assign values to another attribute, please try the following:
1. Click a container or an entry| Right Click Context menu select - Export - Update Statements| Provide the file Name | Click on Browser button adjacent to Returning Attributes and select attribute (for e.g telephoneNumber)| Click Finish
2. Open the exported file using Textpad| Globaly replace the attribute (telephoneNumber) to the new attribute name| Save the file
3. Click Import|SQL from the top tool bar and Click the Start Importing.. Button.
4. Refresh the entry, the new attribute should have the old field value.
Note: Please test with one entry and in a test environment before applying in production.