Starting in Symantec Endpoint Protection Manager (SEPM) 14.3 RU1 unchecking the "Maintain existing client features when updating" option in Upgrade Groups Wizard does not allow you to proceed.
Steps to reproduce:
1. Click on Admin
2. Click on Install Packages
3. Click on Upgrade Clients with Package
4. Click Next
5. Select a package then click Next
6. Select a group then click Next
7. Click Upgrade Settings and uncheck the box Maintain existing client features when updating
8. Click OK
It's at this point that clicking OK does nothing and you are not able to proceed unless you re-check the box.
This issue is fixed in Symantec Endpoint Protection (SEP) 14.3 RU2. For information on how to obtain the latest build of Symantec Endpoint Protection, see Download the latest version of Symantec software here.