Both the father (Howard Stark) and the son (Tony Stark) represents the same character development in MCU productions. Since MCU casted Dominic Cooper for the young Howard Stark position and Agent Carter production followed the same line, we can assume they have the same timeline.
Dominic Cooper as Howard Stark in Captain America: The First Avenger
Dominic Cooper as Howard Stark in Agent Carter
As mentioned in the comments and the OP's question, he changes through time and becomes a different man. Though, I do not see a man who is "reserved and emotionally distant."
He was a funny man. He did weird stuff when he was younger. But he was still funny when he got older.
I believe he saw himself in his reckless son, Tony. So he was being diffucult from time to time when it came to dealing with his son. That's why their relationship was always pictured as something sad to the audience. Because it was from Tony's POV.
As we see in CA:CW
And it was from Tony's POV. (This scene made me cry...)
Tony always saw his father someone unreachable. He even confessed this to Nick Fury.
And he (Tony) was always compared to his father in his professional life (and maybe in his private life as well).
But I believe, they loved each other in a weird way.
I don't think he became a colder person. What he was trying to do was trying to prevent Tony from making the same mistakes he did when he was younger. He failed in a genius - playboy - billionaire - philanthropist way. Because his son became the exactly same person who he was.
And after watching CA:CW, I belive Tony is following his father's footsteps even if he doesn't realise that. He tries so hard to protect the people he loves, he even gives up his Tony Stark-ish manners and accepts his faults and errors.
So bottom line is, the character development of Tony Stark shows possible reflections of how it may have been for Howard Stark in MCU. Losses of loved ones and hardship of life might have changed him, like the way it changed his son Tony.
Photos and gifs are from various sources.