What Happened When You Opened The Database In Startup Upgrade Mode

Basically startup upgrade will open the database by setting the below parameters in memory.(not for spfile)

ALTER SYSTEM enable restricted session;
ALTER SYSTEM SET _system_trig_enabled=FALSE SCOPE=MEMORY;
ALTER SYSTEM SET _undo_autotune=FALSE SCOPE=MEMORY;
ALTER SYSTEM SET undo_retention=900 SCOPE=MEMORY;
ALTER SYSTEM SET aq_tm_processes=0 SCOPE=MEMORY;
ALTER SYSTEM SET resource_manager_plan='' SCOPE=MEMORY;

It will be just a normal database startup, but it will make an environment for migrating the version.

Alert log will show all these details:

Starting ORACLE instance (normal)
LICENSE_MAX_SESSION = 0
LICENSE_SESSIONS_WARNING = 0
Picked latch-free SCN scheme 2
Autotune of undo retention is turned on.
IMODE=BR
ILAT =18
LICENSE_MAX_USERS = 0
SYS auditing is disabled
ksdpec: called for event 13740 prior to event group initialization
Starting up ORACLE RDBMS Version: 10.2.0.3.0.
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Mon Mar 25 19:54:01 2013
Database mounted in Exclusive Mode
Completed: ALTER DATABASE   MOUNT
Mon Mar 25 19:54:02 2013
ALTER DATABASE OPEN MIGRATE
Mon Mar 25 19:54:02 2013
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Starting background process MMNL
MMNL started with pid=12, OS id=2536
Mon Mar 25 19:54:14 2013
ALTER SYSTEM enable restricted session;
Mon Mar 25 19:54:14 2013
ALTER SYSTEM SET _system_trig_enabled=FALSE SCOPE=MEMORY;
Autotune of undo retention is turned off.
Mon Mar 25 19:54:14 2013
ALTER SYSTEM SET _undo_autotune=FALSE SCOPE=MEMORY;
Mon Mar 25 19:54:14 2013
ALTER SYSTEM SET undo_retention=900 SCOPE=MEMORY;
Mon Mar 25 19:54:14 2013
ALTER SYSTEM SET aq_tm_processes=0 SCOPE=MEMORY;
Mon Mar 25 19:54:14 2013
ALTER SYSTEM SET resource_manager_plan='' SCOPE=MEMORY;
replication_dependency_tracking turned off (no async multimaster replication found)
Completed: ALTER DATABASE OPEN MIGRATE