Apparently Larry Nemecek will not be returning as McCoy in Episode 3. He has publicly stated that real life business and conflicting schedules are too time intensive to allow him enough time to play McCoy. I'm not sure who will replace him.
One small observation. Did anyone else note Erin Gray's uniform looked too big for her especially in the sleeves? Nice touch they added a starburst insignia to her uniform.
A note on characterization. It's a fine line between homage and parody. No one can replace the cast that defined these characters for generations of viewers. I suppose it's a matter of channeling elements of the original performance in such a way so they seem to emerge naturally within a new actor's interpretation. Chris Doohan's take on Scotty is not exactly like his father's as it's filtered through Chris' own distinctive character. This is similar to Vic Mignogna's take on Kirk. Neither are or can be exactly like the originals, and even if they were it's questionable if the performances would feel natural.