Move Oracle Database to New Server with Dataguard in place
Create a full backup, then restore it to new server and update the dataguard configuration. Problem is, SCN in Standby database maybe
  different than New Primary database due to old Primary database is
  still running. If this happen, can i apply log from old Primary to New
  Primary? I'm not sure..
  


You can have multiple standby databases for a primary database. Create a full backup and restore it as a standby on the new machine, update the Data Guard configuration and keep that standby too in sync with archivelogs. When the time comes, simply perform a switchover to this new standby database.


  
  Switch the existing Standby database to be Primary database and create a new standby database in new server. Then switch back Primary
  role to new database in new server.
  


No, this is just unnecessary risk and extra work.