Wouldn't it be easier to have two separate login pages on the same App? (One authenticates with HBO itself & the other with Comcast/Time Warner et al.) Once authentication is done, you get to see the same content.
They could have removed HBO Go on Apple's devices when Now launched & then gradually remove Go, as Now spread to remaining platforms.
This sounds like what they should have done. Replace the app with "HBO Now", but keep the HBO Go branding, app store presence and icon. Make it a major version update.