This will work but newly generated chunks will be different, leading to sudden biome changes and cliffs between existing and new terrain, much like these examples (in these cases I actually loaded a modded world in vanilla, with the seed remaining the same, but the same applies to a different seed because each seed has different world generation even if the version remains the same):
In this case the modded version doubled the depth of the ground, leading to giant cliffs down to vanilla terrain, which was otherwise the same (much smaller cliffs will occur in worlds where the seed changed, though you could have a mountain biome cut off by an ocean, or a custom world which will take on the terrain of the new seed, e.g. a caves world type changing to floating islands):
The only thing that will change in already generated terrain is the locations of slime chunks (where slimes spawn underground), which are determined using the current seed, and for versions between 1.6.4-1.12.2, where the game thinks structures like strongholds are located (i.e. eyes of ender will lead you to where the game thinks they are based on the current seed and witches won't spawn in witch huts) if you don't copy over the structure data files in the data folder, which can also cause structures to generate in otherwise invalid locations, as shown in the second example above (the game saves structure data for all structures within 8 chunks of generated terrain and vanilla read the saved structure data from the modded version, placing them at impossible altitudes). Note that the data folder also stores map item data, which will cause maps to break unless you include the "idcounts.dat" and "map#.dat" files). Worlds in versions older than 1.2, or upgraded to newer versions, will also take on the biomes of the current seed (snowy biomes may have rain and vice-versa since it wasn't saved with the world until then, much like structure data prior to 1.6.4).