John,
1.What you are looking for is updating the directory numbers of the phone from the extension of 1xxx to 3xxx correct.
2. Is there extension mobility iinvolved any which way?
For 1.
A possible workaround is as follows to Export the Existing Phone
Configuration, and re-insert them after making manual modifications to the .CSV File.
The detailed steps are as follows:
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1. Cisco Unified CM Administration---->Bulk Administration ---> Phones
---> Export Phones (All details) by using narrowing down the IP Phones per Phone Model.
2.Run the Job, and save the .txt file created with the results of the
Export. Open the file in a CSV format.
3.Modify the Directory Number column manually.
4.Deleted the Exported IP Phones from the Call Manger.
5.Go to Bulk Administration-----> Upload/Download----> Upload the
modified CSV file.
6.Bulk Administration-----> Phones----->Insert Phones (All details)
and select the uploaded the CSV File.
Test it with a 2-3 test phones and see if it helps.