Hi Rick,
license renewal will be mandatory for an upgrade. Make sure "service check date" in the new license file is up-to-date.
Since TMOS v11.4 the so called policy rules were introduced to replace the former http classes.
The migration script will translate http classes into policy rules automatically. But the result needs to be tested carefully.
As written by Walter, changes in default profiles (i.e. tcp, fastL4 etc.) may get lost.
It´s always best practice to use the default profiles as they are and use them as parents to customized profiles.
With an update like this you will also get a new big3d. In case you have GTM in your infrastructure look after iQuery communication issues.
My summary is primarily about LTM aspects. Have a look at the release notes regarding other modules in use, i.e. ASM, APM etc..
Thanks, Stephan