Pokémon Legends: Arceus' Grand Underground vs. BDSP's?
If it's indeed in Pokémon Legends: Arceus, then the Grand Underground is unlikely to be a copy. The whole premise of cypress is that it's still the god of the past, so an older, more primitive version is a greater possibility. Exactly how similar and different it will be - if it even appears in the game - remains to be seen, but it may have some commonalities with the version that fans are familiar with. Likewise, some may be completely different.
The similarities between the two lie mainly in their structure and environment. Pokémon Legends: Arceus has revealed something out of place in terms of technology, but the world does seem to portray its wild and original themes faithfully. The six areas of the Grand Underground each have different biomes and hideout types, so they are perfect for areas players can explore. Since they can also only be unlocked by advancing the story and reaching certain towns, cities, and routes in Sinnoh, different parts can also be unlocked around the same areas they are in the BDSP, or at least their equivalents. It's impossible to predict whether the floor plan of each area will roughly translate to the new game.
The discrepancy between BDSP's Grand Underground and possible Hisui equivalents may come down to human interference. Hideaways showcase the open-ended gameplay and world of Pokémon Legends: Arceus so they mirror the entire cave system. Emerald's Cave may not have wooden stands, lights, and clean corridors, especially since the game seems to be very focused on exploration. The themes of the more natural Sinnoh and unrefined areas are likely to be well known in the underground caves. Snaking caves, twisting tree roots, gleaming crystalline structures and hidden underground grottoes deep beneath the surface are more likely than stone corridors. Pokémon Legends: Arceus has a chance to show players what the Great Underground looked like before humans started digging the orbs.