Isekai assuming we're going off "another world" definition one could argue this is another world as the timeline was altered the moment they awoke as their younger selves as the timeline has changed, or a little further back if you wanna get technical about it. Some existing media that supports this are Sliders and Back to the future.
In Sliders the main characters travel to alternate realities by way of a handheld device that periodically opens portals to other realities. Usually these realities are very different because it wouldn't be very interesting otherwise to have them change to a world where a brand of nail polis is slightly off color. There was an episode where there were harder to notice differences but that's not as relevant as the episode where they travel to the world where the time stream was slightly off and he met a version of his younger self. Not time travel to be clear the episode was NOT time travel, just happened to be 20-30 years behind.
Back to the Future, a more well known Time Travel show follows a character traveling back in time and on one instance the future before returning to his personal time frame. Each of his outings has consequences and effects on his past reshaping his present into new and different realities. Accepting these different timelines as different realities allows to recognize Back to the Future as loosely Isekai, Steins Gate too.
Basically this is traveling to alternate realities in the style of Back to the Future and Steins Gate.
Side note, there's a lot of old stories and anime that are technically Isekai where the term just wasn't in the common vernacular, it's still pretty restricted to the weeb crowd though.
Seriously though, a lot of stuff counts like "Escaflowne" and "El Hazard" and then there's western media like one of the parts of "Heavy Metal" where a green orb whisks away a nerdy dweeb and reincarnates him as BEFF MC LARGE HEUGE in a fantasy setting voiced by a young John Candy. The intro for Den is actually VERY Isekai.
The list escapes me at the moment but there's a lot of old western fantasy movies and animation like "The Flight of Dragons" and "Dungeons and Dragons" that fills the bill.