Another HR representative says I am merely allowed to add the child on as a dependent to my existing plan and my coverage would change from 'Employee + Spouse' to 'Family'.
This was the case for us when we had our first child (federal gov't employee.) It's a qualifying life event, but what you're allowed to do is related to the kind of event it is - you're allowed to switch to family coverage because of your +1, but not insurers. You'll be able to change insurance plans during open enrollment.
HDHP would have been the wrong move for us too. I think we'll be waiting til we're done having kids and they all get out of the very young child age and into school.
Congrats! I'm also due in August with #2.