FTP Test

I switched from a dumb trainer to a smart one. Do I need to re-do my FTP test?

That depends. Are you doing Zwift Workouts or are you just riding?

I want to do the Zwift workouts.

Do an FTP test and after you are done you will have to manually change your current FTP to the new FTP.

You will need to manually do it since Zwift only updates it occasionally so it may not reflect what your got on your new FTP test. 

Do i change it in Settings? I cant seem to find where i can manually change it. Thanks!

Yes, you can access it from Pause Screen. When you stop riding you will see a Menu button on the bottom left of the screen, click on it and click on Settings.