Hello @PingWraith
So if I’m understanding you, and please correct me where I may not be, you have a low res mesh with high res displacement detail in the form of a displacement map, but you do not have access to the high res mesh that originally produced that displacement map. You would like to transfer the displacement detail to a higher poly version of the mesh.
The real problem here is that game assets probably contain optimized, triangular geometry that may not subdivide very well, and may not receive detail very well either. If you had access to the original high res mesh, this process would be very easy. You would simply import the high res mesh, remesh it with ZRemesher or manually into a lower poly form with clean quad topology, subdivide it as needed, and project the detail from the original onto the remeshed using one of the various methods for doing this. However, in this case you need to use that geometry because that’s what the UVs are created for.
You might be able to get decent results subdividing the mesh, but it will probably require extreme subdivision. You could bring the mesh into Zbrush, subdivide it as necessary, and apply the displacement map to the mesh. Be sure to activate the Mode button, otherwise the displacement will only preview as a bump effect. You can then apply the map detail to the mesh as real geometry. The preview may not match the end results exactly, so it may take some trial and error.
Depending a lot on the particular situation, this might work ok, but I’m not making any guarantees–you are definitely trying to paddle against the current.
Once you have a high res version of that mesh with the displacement detail baked in, you would remesh it (or a duplicate) as I described above, and project the detail onto a cleaner quality mesh with quad topology. This is the ideal topology for working in Zbrush, and will provide the best results for sculpting and projection. There will almost certainly be clean up work involved here.
Good luck! 
