What are the risks of upgrading to Snow Leopard (if any)?

As with any major change to an OS I'd recommend taking a backup of the machine before the migration. TimeMachine is the Apple's built in backup software. If your upgrade goes poorly you'll be able to restore from a TimeMachine backup without supplying OS media. read more

This doesn't mean that Snow Leopard is suddenly supported again. Rather, it allows Apple to continue offering modern OS X versions to the Snow Leopard users whose Macs can run newer versions. You can upgrade directly from Snow Leopard to any newer version of OS X, including the current El Capitan, and the Mac App Store is the main delivery method for those upgrades. read more