Is there a way to convert a Skyrim mod into an SSE mod?

I heard that mods made for the original version of Skyrim don't work with SSE, and I was wondering if this was true and why that's the case. Is there a way to convert Oldrim mods to mods that can be used with Skyrim Special Edition easily? And could you tell me what the steps are?

I want to convert a monster replacer that replaces the wolf's textures and 3D model, but it's only available for the original Skyrim.

Solution 1:

There is.

But different files of mods need different conversion methods.

• meshes
  Meshes (and materials) need conversion. To do this, download SSE NIF Optimizer from here, and run the files of the old mod through it.
  If the mod uses a .bsa, download BSA Archive Extractor here, and extract the files first, as SSE cannot read the old .bsa files (you might need to repack them afterwards using the specific SSE BSA file format).
  For your wolf replacer mod, this step is probably the only one you'll need, unless it comes with an .esp.

• .esp/.esm
  .esp files need^* to be converted (from 'Form 43' to 'Form 44').
  This process is really simple: open the Creation Kit (you can download the CK through the Bethesda Launcher, which you can download at the bottom right of this page), load the old .esp (and dependencies), wait for it to complete loading, and save.

  • In case of .esm's (master files), you first have to change them to .esp to be able to properly open and convert them in the Creation Kit.
    To do so, download SSEEdit from here, open the .esm file of the old mod, and go to the File Header of the file in the left field, then to the Record Flags line in the right field. Uncheck the ESM flag there. Save the file (use backups), rename the actual file from .esm to .esp, and follow the instructions for .esp's.
    Afterwards, you can rename the file, and reset the ESM flag in SSEEdit.

• scripts
  Here it might become more problematic: the majority of scripts can be used by the updated engine without any problems, but some make use of external files, like SKSE or other .dll files (e.g. Grimy), and refer to commands that could be missing or renamed in their equivalents of SSE. They can probably be recompiled, but I've never tried that myself.
  In case you find an Oldrim mod that needs an additional plugin and has not been converted, I think it's best to ask after that specifically.

For a more detailed description of these steps, go here.

^{* Technically, they don't, but re-saving them in the CK optimizes them and might prevent save game bloat.}