The trainers would likely be heartbroken. They would have grown attached to their Eevees over the years, and knowing that their time together is so limited would be really tough. They might become overprotective or try to make every moment count.
An Eevee would probably seek out adventures that allow it to explore different terrains. It might want to visit a big forest, a high mountain, or a deep cave. Since its time is short, it would be eager to experience as much of the Pokemon world as possible.
Well, other trainers would probably be both jealous and in awe. Some might try to befriend Ash just to get closer to the legendary pokemon. Others, especially those with a competitive streak, would see Ash as a huge threat and might be less friendly towards him.
Well, it could lead to some really interesting scenarios. They might gain the powers and abilities of the Pokemon they turn into. For example, a trainer who turns into a Pikachu could use Thunderbolt. And they would have to adapt to living as a Pokemon, like finding food in the wild and dealing with other Pokemon in a different way.
One interesting plot could be a trainer who turns into a Pikachu and has to navigate the Pokemon world from a different perspective. They might struggle with their new small size and electric abilities at first, but then learn to use them to their advantage in battles.
Young trainers would be excited. They'd see it as a great opportunity to start their journey with a unique Pokemon like Eevee. They might rush to the adoption place, full of enthusiasm. Some experienced trainers, on the other hand, might be more cautious. They'd want to make sure they can handle another Eevee in their team, considering their training style and the other Pokemon they already have.
In the hospital setting of a fanfiction, trainers would interact with their Pokemon in different ways. Some trainers could be using their Pokemon as emotional support for themselves while they visit sick friends or family. A trainer with a Snorlax might just sit beside it and feel comforted. Others would be guiding their Pokemon to do good deeds. For instance, a trainer with a Psyduck could be trying to get it to use its water - based attacks in a controlled way to help clean up small messes. Trainers would also be teaching their Pokemon proper hospital etiquette, like not running in the hallways.
One possible story could be that a trainer has too many Eevees due to some unexpected breeding or a special event. They can't take care of all of them, so they put them up for adoption. Another idea is that a Pokemon rescue center has a box full of Eevees that were found abandoned in the wild, perhaps because their previous trainer couldn't handle their care anymore.
One negative impact is that it objectifies female trainers. Instead of focusing on their skills, abilities, and their relationships with Pokemon, it reduces them to objects of seduction. Another is that it can make the Pokemon fandom seem less family - friendly. Pokemon has a large following of young fans, and this type of fanfiction can be inappropriate for them to come across.
Another aspect could be the change in the social hierarchy within the pokemon world. If trainers are reversed, perhaps they are now seen as more of a sidekick to the pokemon. This could lead to different power dynamics and interactions not only between pokemon and trainers but also among the trainers themselves. They might be competing to be the best pokemon's sidekick rather than having the best pokemon.
Well, it varies a lot in fanfictions. Sometimes trainers have to go to school in the morning and have their Pokemon training in the afternoon. Or it could be that they attend a special Pokemon training school all day long.