The theme seemed to be being replaced by the young in this episode that tied Roger's increasing lack of role in the agency, Peggy's finding the next her and Betty's depression all into one episode. Don and Harry's trip to the Rolling Stones also showed them out of place with the teeny boppers. What is funny is Harry is still relatively young and is still out of place in that world.
Betty got the main story as she is now getting fat and seems to have thrown in the towel being married to Henry. He is presented as pretty loving and doesn't seem to mind her weight, which also might explain why she is getting big. He is too accepting and he does not seem to challenge her in the right ways to make her a better person. Last season he did call her out on firing Carla and seemed to put his foot down, but Betty seems stuck as wanting to be Don's wife. I don't think she wants Henry, although he is better than she had a right to expect being divorced with three kids to another man. She wanted to be Don's wife and be a team with him, but his constant cheating made that unbearable. She is stuck in the past.
Unlike Betty who chooses to be stuck in the past, Roger doesn't like it, but it doesn't look like he can do much about it. Pete is practically throwing him out the door and steals the credit for Mohawk. He is someone who doesn't want to retire, but may be shown the door anyway, especially in the youth movement 60's.
Don is increasingly out of his element in the youth culture too, being more fatherly than Don Draperish to the young girls at the concert. He married Megan because he was attracted to her youth, but it seems to be slowly dawning on him that instead of making him feel young, its making him feel old.
Finally, Peggy hires a new copyrighter and she is warned to not hire the talented guy, but someone mediocre to protect her turf, but she ignores the advice. Stan's warning may be like Faye's to Don last year about being married in a year. Mad Men likes prophetic pronouncements so the new guy might end up being her boss.