There’s no reason you shouldn’t be able to control the volume. However you can’t just remove one sound bar and install another. You’ll still have to follow the remote setup in the app. If you run into difficulties please post back for assistance.
Edit: Also keep in mind that you are moving from optical to HDMI. The Arc Ultra will work with Optical but then you might as well stick with the Playbar.
The best way to transition to your new setup is to remove via the App the sub and surrounds from the Playbar and then the Playbar itself.
Setup the Arc Ultra first and get everything working including the remotes. Then add your surrounds and last your sub.
Also,Be sure to add the Era 300’s as separate rooms before attempting to add them as surrounds. The room names can be fictitious.