Patch Management Group

do I need to disable all existing software update policies prior upgrade? also do I need to upgraded endpoint symantec agent and plugins before patching?

You don't need to disable policies.

7.6 release supports legacy clients, so you don't need iimmediately upgrade SMA agents and plug-ins to continue patching endpoints.

Be sure to review the release notes in detail for instructions on upgrading.   Since you are upgrading from 7.1, be sure to follow the recommended upgrade path which means upgrading first to IT Management Suite 7.1 SP2 MP 1.1 (v11) if you are not already at that level.  I'd also recommend installing the latest 7.6 hotfix (currently HF4) but you may want to review the release notes for that first to see the list of fixes and known issues related to the products you are using.

In my experience, patch management has handled upgrades very well.  Most upgrade challanges I've had have been around Deployment solution.

7.6 documentation index: http://www.symantec.com/connect/articles/itms-76-documentation

TIP : change the Communication Policy to "Check In" every 8 hours (you may want to increase this if you have a lot of clients).

This way when the server is upgraded agents get rolled out in a slow but steady pace. 

Also if you run into problems - you have a few hours to fix them. If you still have problems you can turn off communication completely.

We set all our agent install and upgrade policies to a schedule vs. the default of "Once". Our upgrade form 7.6 HF 2 to 4 required no intervention in the agent upgrade process.

After 2 days we returned to our normal 2 hour "Check In".

Per your question - the agents only get upgraded after the server.

One addition in 7.6 HF4 is that it includes full support for Windows 10.