How to change vanilla structures/jigsaws

How To:

1. Your structure needs the exact same name and ID as the structure you're replacing
2. Ancient City is a jigsaw structure, so ensure the structure blocks are all in the proper places and recall the proper target pools/structures that the original structure uses
3. Ensure that you put the structure in the exact and proper structure folder where the vanilla one is located in your mod directory.
   1. data > minecraft > structure > ancient city > (identify what part of the ancient city/pool the segment is in and create that folder for it

Troubleshooting:

1. If you have done all of this, go back through and DOUBLE CHECK that your jigsaw blocks have all the CORRECT data

you will have to manually create the files in your mod directory.

Here is the directory tree for Ancient Cities to give you a reference to what you should add to replace specific structures

Each /file contains specific .nbt structures, figure out which directory path the structure you are replacing uses then you know where to place your structure in your data/minecraft/structure folder. 

If you are using 1 structure to replace numerous structures, you will have to rewrite the pool.json file for the target pool you are affectively changing. Pool.json files are found in the data/minecraft/worldgen/template_pool file

• /data
  • /minecraft
    • /structure
      • /ancient_city
        • /city
          • /entrance
        • /city_center
          • /walls
        • /structures
        • /walls
    • /worldgen
      • template_pool
        • /ancient_city
          • /city
          • /city_center
          • /walls

I recommend delving into a minecraft version folder to find these so you know exactly what you are looking for and replacing. You can find what you are replacing with the "/place template" minecraft command as well as what folder the structure is in.

IF that does not work, you will likely have to just rewrite the pool.json file to include your structure and not include the vanilla structures

rewriting the pool.json file doesn't seem to work so that might require actual coding.