If this is aiming to be shown in 4K, then they might keep the old episodes at the original AR.
Somewhat unrelated but I'm noticing that HBO is showing more and more films in their original AR, as opposed to pan-and-scan them like they've done forever. The whole argument about black bars is as antiquated as the old tube televisions that facilitated that point to begin with, and hopefully that's what HBO is getting hip to now.