Why would Ash use his old Pokémon fanfiction?

He might use them because they are his tried and true companions. Ash has a strong bond with his old Pokémon, and in the Kalos region, he wants that familiar support.

It could be Pikachu. Since Pikachu has been with Ash from the very beginning and is his most iconic Pokémon. It's very likely that in Sinnoh fanfiction, Pikachu would be frequently used as it has a strong bond with Ash and is also very powerful.

Ash can use old fanfiction to better understand different perspectives within the Pokémon world. Fanfictions might have explored the lives of Pokémon in ways not shown in the main series. By using these ideas, Ash can become more empathetic towards Pokémon and form stronger bonds with them. For example, if a fanfiction delved into the feelings of a shy Pokémon, Ash could use that knowledge to help such Pokémon in his journey.

At first, his Pokémon would be startled. They've always known Ash as a human. But then, they would start to adapt. Some Pokémon, like Squirtle, might try to play with Ash in his new form, thinking it's a new kind of game. However, his more loyal Pokémon, like Greninja, would be determined to find a way to turn Ash back. Greninja might use its special bond with Ash to try and communicate with him on a deeper level in this new form, to understand what happened and how to reverse it.

He could be using it to improve his writing skills. By analyzing his old fanfiction, Ash can identify areas where he has grown as a writer and areas that still need improvement. For example, he might notice that his descriptions of Pokémon battles were a bit simplistic in the past and work on making them more detailed and exciting in his current writing.

Some of his Pokémon might be very confused and sad. Pokémon like Pikachu, who has been with Ash for a long time, would probably miss the adventures and battles they had together. Pikachu might try to cheer Ash up and convince him to change his mind.

Some of his Pokémon like Pikachu might be very protective. Pikachu would stay close to Ash at all times, being extra vigilant in case any danger comes while Ash is in this new form. It would also be curious and might try to play with Ash's new fairy features, like his wings if he has any.

His other Pokémon like Charizard would be completely bewildered. Charizard might initially think that this Eevee is just some random Pokémon near its territory. But when it senses something familiar about the Eevee, it would be hesitant and curious. It could try to communicate in its own way, like using small bursts of fire to get the Eevee's (Ash's) attention. The Pokémon that Ash has a closer bond with would be more determined to figure out what's going on and how to help him.

His other Pokémon like Bulbasaur and Charmander would also be startled. They might gather around Ash - Zoroark and sniff him out to make sure it's still him. They could be a bit hesitant to follow his commands at first because they're not used to his new form. However, as they see that Ash's spirit is still there, they would gradually start to trust and follow him again, and they would work together to figure out how to turn him back.

In some fanfictions, Ash can touch Pokémon to show a deeper bond. It might be a way for the author to create more emotional and physical interactions between Ash and his Pokémon pals, different from the normal anime where physical contact like hugs is not as common.