This is highly variable. I was also "told off" when visiting my wife in Y (skycouch row with babe).
I did not have "infant in arms" on my ticket and they would not swap the infant over or let her in J which I think was a little harsh (secretly happy).
I am really surprised this was allowed long haul (seat swapping with J/Y), I think its pretty cheeky and wouldn't even attempt it.
Friends of mine (work pays for J) but spouse in Y have had 0% success in doing this TT over 3 attempts.. they stopped asking.