Pikachu might be very confused at first. When it sees 'Ash' (really May in Ash's body) acting differently, it could be hesitant to follow commands as usual. For May's Pokemon, they might be surprised to see 'May' (Ash in May's body) suddenly having a more battle - oriented attitude. For example, May's Beautifly might be confused when 'May' wants to train it for battles instead of focusing on its appearance for Contests.
Since Ash has a closer bond with his Pokemon through battles, May in Ash's body might find it challenging to maintain that same level of connection. Her different approach to Pokemon handling could make some of Ash's Pokemon a bit resistant at first. On the other hand, Ash in May's body might introduce a new way of training to May's Pokemon that they've never experienced before, which could either be exciting or a bit overwhelming for them.
I think Ash would be angry as well as hurt. He's a passionate trainer and his friends mean a great deal to him. He might challenge May in a Pokémon battle, not to defeat her but to make her see how wrong her betrayal is. And during the battle, he would keep asking her to come back to their old friendship.
One interesting plot could be that Ash, now in May's body, tries to enter a Pokemon Contest. He has to quickly learn the skills and grace that May has, like how to style the Pokemon in a more appealing way for the judges. Meanwhile, May in Ash's body decides to enter a Gym battle. She struggles with Ash's more straightforward and battle - focused approach compared to her Contest strategies.
Ash's Pokemon would likely have mixed reactions. Some like Bulbasaur, being the gentle and observant one, would be quick to accept Sabrina. It might even enjoy the new psychic powers that Sabrina could bring into their group. However, Squirtle might be a bit mischievous at first, trying to test Sabrina's patience. For Sabrina's Pokemon, Alakazam, being highly intelligent, would sense Ash's good intentions towards Sabrina and would support the relationship. Her other Pokemon would follow Alakazam's lead.
An interesting plot could involve a strange Pokemon ability that causes the body swap. As they are in each other's bodies, they are forced to travel together to find a way to reverse it. Along the way, they encounter other trainers who notice the strange behavior and they have to come up with excuses. This leads to all sorts of adventures and misunderstandings as they explore each other's lives in a whole new way.
His Pikachu would probably be very supportive. It would stay by his side as he studies, maybe even help him in small ways like fetching medical tools. It trusts Ash completely and would be happy to see him pursue something new.
Another theme is adaptation. Ash has to quickly adapt to the new body's physical abilities, like different levels of strength or agility. This often leads to some really interesting scenarios where he has to figure out how to battle or move around in a completely different way. For example, if he swaps with a smaller trainer, he might have to find new strategies for catching Pokemon. Also, there's often the theme of friendship. His friends might not recognize him at first, and he has to prove that he's still Ash on the inside, which shows the strong bonds of friendship in the Pokemon world.
If Ash was in Serena's body, his relationship with her friends like Bonnie and Clemont would change. Bonnie might be a bit confused as Ash's more boyish and impulsive nature shows through Serena's body. Clemont might be surprised at some of the battle - like decisions Ash makes when dealing with things like helping around the lab. On the other side, if Serena was in Ash's body, her relationship with Ash's rivals like Gary would be different. Gary might be intrigued by the more refined and composed 'Ash' he now sees.
Ash would probably be very shocked and hurt. Pokemon are like his family, and for Misty to steal one would go against everything he stands for.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.
May could be completely surprised. Given that Drew has always been a bit of a rival and a bit aloof at times, she might not have expected his confession at all. So she could just stand there, mouth agape, not knowing what to say at first.