How to ensure the quick switch from Log shipping to Mirror in SQL Server 2012
Below would be my steps :


disable all backup and copy jobs on primary and secondary.
manually run the copy and restore jobs couple of time to ensure that all the log backups are applied to secondary.
remove logshipping using GUI
restore database on secondary using NORECOVERY - if db is readonly or standby mode.
Enable mirroring using scripts or wizard


this is your logshipping to mirroring conversion.

Make sure you take regular log backups on the principal else your log file will grow.