Upgrade issues (10.50.1600.1 to 10.50.4000.0)?

Related to http://social.msdn.microsoft.com/Forums/sqlserver/en-US/49492155-c22c-46fe-9e92-b80dd9d95ffb/upgrade-sql-server-from-105016001-to-105040000?forum=sqlsetupandupgrade

Any known issues with this upgrade? I'll get a backup of the server first, but as these are live servers with existing dbs on them, will their structure be changed in ways that could stop them from working with our apps?

Also, roughly how long does the upgrade take? Is it just running an exe? Reboot necessary? More?

Thanks,

Scott