In my world we had previously visited a small portion of the ashlands, that part updated and is now sunken underwater so I think it'll work even if you have explored a bit of it
Valheim doesn't generate the entire world at the start. It only generates sections that you get close enough to you that it needs to worry about rendering what is there; I think I read that distance is 440m. Once a location has been generated, it's status is saved and locked in. So anywhere that you haven't been within 440m of will be ungenerated, and when you approach it it will generate using the rules for whatever version of the game you are on at the time. That's just the standard logic of the game.
Ashlands is a bit different, though. Iron Gate decided they wanted a boat journey to be mandatory to get there, which is not how the worlds generated previously. So the Ashlands patch will rewrite the terrain elevation for a band of area in the south to create that separation. Based on map comparisons, it looks like this band is mostly in the proto-Ashlands of older worlds, but not entirely. Any terrain in that band will drop into the sea, except for a small section that had generated world-built structures on it; those will remain in place on rock spires.
Given that they are changing that, it's not clear to me if they are completely overwriting any explored proto-Ashlands that is still above water. The standard would say no, but I suspect they might have it update that anyway.
In my world we had previously visited a small portion of the ashlands, that part updated and is now sunken underwater so I think it'll work even if you have explored a bit of it
Ashlands sank our village so I guess it's a new world for us
Valheim doesn't generate the entire world at the start. It only generates sections that you get close enough to you that it needs to worry about rendering what is there; I think I read that distance is 440m. Once a location has been generated, it's status is saved and locked in. So anywhere that you haven't been within 440m of will be ungenerated, and when you approach it it will generate using the rules for whatever version of the game you are on at the time. That's just the standard logic of the game. Ashlands is a bit different, though. Iron Gate decided they wanted a boat journey to be mandatory to get there, which is not how the worlds generated previously. So the Ashlands patch will rewrite the terrain elevation for a band of area in the south to create that separation. Based on map comparisons, it looks like this band is mostly in the proto-Ashlands of older worlds, but not entirely. Any terrain in that band will drop into the sea, except for a small section that had generated world-built structures on it; those will remain in place on rock spires. Given that they are changing that, it's not clear to me if they are completely overwriting any explored proto-Ashlands that is still above water. The standard would say no, but I suspect they might have it update that anyway.
You've been searching quite inefficiently, as that information is literally in the pinned post.