Timon and Pumbaa were turned back to normal by Rafiki. They were disappointed to learn that Simba was no longer living in the oasis or even the pride lands. Of course, it wasn't as if they would never see him again. Ultimately, if Simba was happy, they were happy.
Faline chose to stay a lion; it was exciting and new. The biggest reason for her staying was Bambi. Rafiki did offer to make her a deer again but she didn't feel like she had to.
Rafiki gave warning that Scar could not get a hold of a cure, if he was to be exchanged. So he waited to continue to research until he was sure that Scar was no longer a threat. The biggest thing to come from his research was turning Timon and Pumbaa back. Try as he might, he just couldn't ``cure'' Simba and Bambi. He best way he could describe it is that the energy had too strong a hold on them, most likely from his initial attempt to cure them. Eventually, he ended his research and with its end, the gateway closed to everyone except for Simba, Bambi, Faline and presumably, Scar.
As for Scar himself, no one quite knows what happen to him. Rafiki says he never saw him since the attack and certainly not after his ``exchange''. There were several sightings of him both in the pride lands and the forest but nothing's for certain. Some say he reclused into a life of solitude, never letting anyone in. Some say he tried to attack them, as if still a lion. And others say he met his fate as someone's prey. The only thing we can be sure about is that he never accepted that he was a deer.
Simba and Nala had a quiet life in the forest. He was already accustomed to it and had no trouble readjusting. Nala had a short period of adapting to her new surroundings but it was easier with Simba by her side.
Bambi assisted Rafiki with his research. After that ended, he went off on his own (except for Faline, of course). He tried to integrate back into the pride but after Scar's disappearance, a power vacuum had formed and they weren't excepting male lions. He never looked for a reason to hunt but he knew how to if he had to and always knew when to draw the line.
They kept their promises and made visits to their old homes but they were happy in their new ones.