Basically, if you do your homework, 1.6.2.0 to 1.9.2.x is pretty trouble-free in the database upgrade department, if you take care of the Google Checkout thing during pre-conversion (basically remove all Google Checkout module bits and allow 1.9 to reapply its stubs).
Till it got upgraded, the MDN Extension Conflict reporter showed that most of Magento was actually 1.6 on the core module versioning with a scattering of 1.7, 1.8 and 1.9 code versioned modules.
And if you're doing your homework, you've done the upgrade several times on a dev or staging server. And if you've implemented git then you basically start despising hearing about anyone who applies their version upgrade to the live site FIRST before donning sackcloth, ashes and getting religion. So sweet changing DB credentials and branches to back out just in case the live site is a fine spawn of Satan that has unreasonable failure that the staging install on the same exact server never expressed over the 14 times you wiped and retried.
