Per the Symantec Endpoint Encryption upgrade guide, you can directly upgrade encrypted clients using Symantec Encryption Desktop 10.3.2 mp4 or later to Symantec Endpoint Encryption 11.1.0. The upgrade guide and other docs can be found here:
https://support.symantec.com/en_US/article.INFO3452.html
I was in a pilot program here at Symantec for testing the migration from Symantec Encryption Desktop to Symantec Endpoint Encryption, and I have had no issues on my system. Symantec Encryption Desktop will also remain installed, so you can continue using its other features if you have more than drive encryption enabled. The only downside I can see is that it fully disables the Symantec Encryption Desktop drive encryption driver, so you will need to include SEE Removable Media Encryption if you want to encrypt external drives' data.