Step 1: identify your Minecraft version
Write down the exact version of the instance you plan to use. Do not assume a file for a nearby version is compatible.
Step 2: confirm the loader and dependencies
Use only files built for that loader family and install the required dependencies for the same game version.
Step 3: back up and test
Copy important worlds and configs. Launch a fresh test world before opening a long-term save, and keep the previous working instance until you are satisfied.