The recent announcement of the latest Cumulative Update for Exchange 2013 said this about schema changes:
Cumulative Update 9 may include Exchange related updates to the Active Directory schema and Exchange configuration when compared with the version of Exchange 2013 you have currently deployed.
So does CU9 include a schema update or doesn't it? The answer is hidden in the "the version of Exchange 2013 you have currently deployed" part.
First let's take a look at how we can find out what version our schema is. To find the Exchange schema version we need to query the rangeUpper property of CN=ms-Exch-Schema-Version-Pt,cn=schema,cn=configuration,DC=domain,DC=local (replace DC=domain,DC=local with the name of your root domain).
In an Exchange forest with CU9 installed the value returned by this query is 15312. There are several online resources which keep track of all schema versions, for instance this one by Michel de Rooij or this one by Todd Nelson.
So apparently CU7 was the latest update to include schema updates. This means the CU9 will perform a schema update when upgrading from CU6 or older and of course when CU9 is the first Exchange 2013 version you deployed in your environment. If you're upgrading from CU7 or CU9 the schema will not be updated.