If you want to change Sip domain name, you can try to follow these steps:
1) Add support of new SIP domain in DNS.
2) Regenerate and assign new Lync server Certificates.
3) Add the new SIP domain as a support SIP domain in Lync + simple URLs
4) Ensure any changes required for integration with Exchange are made.
5) Ensure any third-party applications dependent on the SIP domains are updated.
6) Test the Lync features with new Lync SIP domain and with different clients and versions.
7) Know the end-user impact and communicate what they have to do
8) Change the SIP address of your users.
9) Decommission old SIP Domain: More than one SIP domain can exist at a time, so there is not urgency, but it is recommended you decommission the old domain once it is no longer being used. Review your certificate Subject Names if you are configuring the new SIP address as the primary SIP address in Lync.
For more details, you can refer to this link:
http://blog.insidelync.com/2013/06/anatomy-of-a-sip-domain
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