Please, hold your applause, your eyes are not seeing things, this question finally has an answer. xD
Now, there are only very few speech changes in Lumiose City. All of them occur when you complete the first Looker Mission (which is to find all five Looker Tickets).
Outside the TMV station:
"I'd guess it's already time to say goodbye." "Remember, I'm just a short TMV ride away."
"I'm back again already!" "...It really is just a short TMV ride away."
Middle of Autumnal Avenue:
"I just want to be able to fly around the world freely! Just like a Fletchling!"
"Wait, you mean the Fletchling in Lumiose... just walk everywhere!?"
Outside of the Poke Ball Boutique
"What would you recommend on Autumnal Avenue? Hmmmmm...?" "Hee hee hee! I'd say the Poke Ball Boutique or the Berry Juice Shop!"
"Thanks to you, lady, I've become a real master of Autumnal Avenue!" "Oh ho ho! Aren't you getting ahead of yourself? You haven't even been to the restaurant yet!"
Woman & Fletchling Outside of the Juice Shoppe
Seems like there are more kinds of juice lately, or is it that I've been hanging in Lumiose a lot?" "Jui-chiii!"
"Do you think my Fletchling would enjoy some premium Berry Juice?" "Chi-chi-chi-chiiirp!"
Outside Lysandre Cafe (still on Autumnal Avenue)
"Lysandre loves the color red, so he even made his cafe red."
"Lysandre Cafe's red decor supposedly represents the owner's burning passion."
"I heard from someone that there are Pokemon in Jaune Plaza." "It's hard to stand in Jaune Plaza and not mimic Pokemon cries, isn't it?"
"Whoa! I hear that there's a Lava Dome Pokemon in Jaune Plaza!" "Hohohoho! It's hard to stand in Jaune Plaza and not want to mimic Pokemon cries isn't it?"
And there you have it - every piece of dialogue in Lumiose City that changes after completing the first Looker Mission.
Hope I helped. :)