Endpoint modification in SQL Server mirroring
There is no need of modifying the endpoints. This is the process:


ALTER DATABASE IICS SET PARTNER OFF (Server2)
Change IP address in Server2
Change IP address in Witness 
Change IP address in Server1
ALTER DATABASE IICS SET PARTNER = 'TCP://SERVER1:Port' (Server2) 
ALTER DATABASE IICS SET PARTNER = 'TCP://SERVER2:Port' (Server1)
ALTER DATABASE IICS SET WITNESS = 'TCP://WITNESS:Port' (Server1)