Hi Debaditya.
1. I think you have to perform the SAP system copy since heterogeneous landscape. if the SAP system (CI & DI ) on same system you can refer the SAP note 1233320. If possible you can check with SAP consultant to raising the medium support ticket about heterogeneous landscape.
2. All the additional SAP App servers uninstall from old domain. reinstalled the same to new domain. For primary application server perform the SAP system copy in the new domain. before that take full offline backup on the old domain
3. Primary DB(DB2) any reference are there in Old domain user id's(SAP SIDADM & SAPServiceSID)?
There no issues in the IP address changes in the SAP landscape. you have to take care the DNS or host enters.
Regards
SS