I've long accepted the fact that this is no longer FFVXIII so I really don't care about the character change, but I'm still excited for the game because I love the idea of an ARPG FF. That being said, I can see why people are upset about this change.
If there's a story reason for this change, I would love to hear it. As far as I know, the suit that Regis wore were very much in line with the lore in the game.
Lucis is the most advanced (with the exception of military advancements) country in the whole world due to the magical protection of the crystals. The country is ruled by an organization that operates similarly to Yakuza/Mafia minus the crime. That's why they wear black suits, everyone who is valued highly in the organization dresses according to that dress code.
Noctis on the other hand, is known to be rebellious (which you can see from his behaviour in almost all the trailers that have come out) so he doesn't wear a suit BUT he still dresses in black. That's why their outfits make sense in the context of the world.
With Regis' change in clothing, we now have a modern country which is ruled by a man who is wearing clothing that looks like it's from the same level of technology as the other four countries. That just seems weird.
There are two conclusions we can come to in this case: everyone in the royal family wears these types of outfits now, meaning that Noctis and his friends are probably fashion outcasts in the whole world now. The second case is that everyone in the city wears modern clothing EXCEPT the King. Doesn't that just strike you as an odd and somewhat unnecessary change?
EDIT: This is all, of course, assuming that the character design wasn't made just for flashbacks

, or some convoluted twist about Noctis' father that was already in the original script.