Installing Exchange Server 2013 CU9 from Exchange Server 2013 SP1

So I'm getting ready to apply the update. I wanted to double check with anyone here who has done the update from SP1 to CU9. Are there any issues I should keep an eye out for? Is the application of the update actually seamless? I found a tutorial on Exchange Server Pro: http://exchangeserverpro.com/exchange-2013-installing-cumulative-updates/ 

Is that tutorial still valid even though it's been a couple years? 

Due to AD schema changes in the other CU's should I worry that my Domain and Forest Functional Levels are still set at Server 2003?

Thanks,