Lots of things could change! Maybe Sam alters events that were supposed to happen or meets people from the past in unexpected ways.
It's hard to say exactly. Sam going back in time could lead to altered relationships, different life paths for characters, or even changes to the entire world as we know it in the story.
Well, usually there are big changes. Maybe they prevent some tragedies or fix past mistakes that affect the future.
Well, there could be all sorts of changes. Maybe he'd get a chance to fix past mistakes or prevent certain events from happening.
Well, in some fanfictions, they go back in time and accidentally interfere with the lives of their younger selves. This creates all sorts of chaos as they have to deal with the younger Sam and Dean's reactions to seeing their future selves. Or perhaps they go back to a crucial moment in the history of the hunter community and end up changing the rules and relationships within it. It could also be that they go back in time to a specific era like the Wild West and have to deal with supernatural threats that were prevalent during that time.
He might have trouble adapting to the past and avoiding changing events that could have unforeseen consequences.
The main characters are Sam and Dean Winchester, of course. But in some fanfictions, Bobby Singer might also be involved. Since he was like a father figure to them, he could be the one guiding them through the time - travel process, or he could be there in the past waiting for them. Also, the characters of their parents, Mary and John Winchester, might show up if they go back to a time when their parents were alive.
She could meet her younger self. This would be an interesting encounter as she could offer advice and guidance to her younger version. Maybe she could help her younger self to be less afraid of her powers.
Winston undergoes a significant transformation. He becomes more resigned and less rebellious against the system.
Well, in this kind of fanfiction, Natsu might use his knowledge of the future to prevent some tragedies. For example, he could stop certain battles from happening or save characters who died in the original timeline. Maybe he would also try to change the power dynamics within the guild earlier on, making Fairy Tail even stronger from the start.
Well, Garrosh Hellscream could be an antagonist in this fanfiction. Even if Anduin goes back in time, Garrosh's thirst for power and his extreme views could put him at odds with Anduin. Garrosh might see Anduin as a threat to his own plans for the Horde. The Nathrezim, those sly demons, could also be antagonists. They are masters of manipulation and could try to use Anduin's presence in the past to their own advantage. And don't forget about the various cults that serve the Old Gods. These groups would likely oppose Anduin as he tries to disrupt their plans to free their dark masters.
One change could be in the strategies used in battles. Older Ruby has more experience, so she could introduce new fighting techniques to her younger self and the team. This might give them an edge against their enemies. For instance, she could show them how to better use their weapons in combination with their semblances.