This might cause you some trouble later down the line depending on the steps you take to resolve the biggest issue you will face, from what I recall but keep your head down and try.
First off copy and paste the world folder in this case the "WdMadrigal" you want (V19) into your "client/world" and resource/world folders, make sure the resource/world isn't res encrypted if so take them out of the res format. (If you don't understand what this means well umm look inside the folder....)
Now inside the resource folder update the Terrain.inc and the MdlObj.inc to handle the new models and terrain, be sure to add them to the client resdata that they belong to (or use Merge).
you should be rather finished other than the crashing you will get from quest items trying to spawn, monsters and NPC's not yet defined inside the resource.... xD easiest way to bypass that little issue would be to replace the .rgn and .dyo from V15 to the new WdMadrigal... Good luck if I forgot or missed anything sorry it's been a rather long time since I've done it.
Oh... thinking about it some places have animations like bahara desert if you do this method some animations might cause you do crash or may not the correct one... also be sure to add the .o3d of the models into the model folders.