Update primary SQL Server in mirrored environment using transaction logs from old environment
When we are ready to go live with the new environment I plan to take another transaction log from the old environment and restore that to the new primary server. Is that possible? Will this be successfully synchronized to the mirror?


You can set up mirroring as normal. But when you want to failover, then use SYNCHRONOUS - High safety without automatic failover.  



using T-SQL : 

ALTER DATABASE &lt;&lt;DB NAME&gt;&gt; SET PARTNER FAILOVER


Doing so, only the role reversal happens and mirroring direction is reversed - principal becomes mirror and vice-versa. Note that MIRRORING is not BROKEN.

Now once you verify everything is OK, then you can break mirroring with below steps :

Turn mirroring off (break mirroring) on the mirror DB.

ALTER DATABASE &lt;&lt;DB NAME&gt;&gt; SET PARTNER OFF


Set the DB to RECOVERY

RESTORE DATABASE &lt;&lt;DB NAME&gt;&gt; WITH RECOVERY


Migrate Logins as well as any sql agent jobs to secondary. You might have to fix orphaned users.

EXEC sp_change_users_login ‘Auto_Fix’ , ‘&lt;&lt;username&gt;&gt;’