I currently get HBO via Hulu, and for example, I can watch ANY episode from The Sopranos. I just added Cable TV from Xfinity, and before I cancel HBO via Hulu and add it to my Xfinity subscription, I'm trying to find out if any of the older episodes of The Sopranos or Game of Thrones require a fee. Haven't been able to get the answer from the reps on Xfinity Chat.
I bring this up because I was surprised to see that there is a fee required for older episodes from a series on AMC even though AMC is a channel included in my subscription.
I'm not going to move HBO from Hulu to Xfinity if I have to pay above and beyond the $15/month to watch older episodes.
I would just leave your HBO subcription with HULU for now. The video quality may be better that way. No need to change what is working for you to watch HBO.
You can check for old posts here to see what people say about HBO through Comcast.