The missing link: Game Engines
So far we've seen no evidence suggesting this remade model statement is remotely true, but I've been withholding some information from you guys.
A model is a representation of points and faces, and can be stored in an exchangeable form, for editing purposes. Yet they can also be stored in a more convenient read only form, for game engines.
And guess what?
Pokémon XY,
Pokémon Sun and Moon,
Pokémon Let's Go Pikachu and Let's Go Eevee, and
Pokémon Sword and Shield all use different game engines.
Pokémon XY's engine was shared with
Omega Ruby and Alpha Sapphire, even sharing model and map compatibility between the two. The model format used for this engine is BCH, similar to A Link Between World's models, too.
Pokémon Sun and Moon scrapped the previous engine, instead focusing on a new one with file streaming, for example. The maps aren't a single mesh anymore, instead certain props get loaded up individually as your player approaches them, or get replaced with imposters the further you go. As for models? Game Freak opted for their own format, the gfmodel format.
Supposedly this engine move was due to loading speed, Z-move animations and performance reasons.
Pokémon Let's Go Pikachu and Let's Go Eevee have a brand new engine.... well is it brand new? It's really a Frankenstein engine. It features code from the Nintendo SDK (used for BotW and Mario Kart 8), it features file formats from Lunchpack (Splatoon 2's engine) and it features Game Freak's own additions, along with Game Freak's new model format, gfbmdl, or Game Freak Binary Model. It actually doesn't share much with the previous gfmodel incarnation, believe it or not.
Could it be then that they're using a brand new engine again for
Sword and Shield? It's very likely that Game Freak has went ahead and removed their Nintendo SDK and Lunchpack dependencies, and along with it, streamlined their gfbmdl format some more. Under this assumption, sure the models could've been remade.
tl;dr What remade models probably mean
Wow, we're finally here in the end. If you've read up to this point, you probably know what the difficulty in bringing all the Pokémon to the new games is.
But for those of you that skipped, worry not! I'll summarize it real quick:
Technology, Time and Money
The new aesthetic style, allowed to flourish thanks to the new technology, implies redoing textures all over again for the entire collection, meaning there's more time and money constraints added to the mix.
A new model format implies reconverting all of these models, skeletons, animations and more along with bug testing every single one of the model converters, loaders, new animations and whatnot. This might sound simple on paper, but I assure you it's not, a lot of complications might pop up unknowingly.
Here's an (hilarious) example of what happens when a new animation and model format is being debugged, from Sony and Marvel's Spider-Man
https://youtu.be/KDhKyIZd3O8?t=2433 (40:33)