How to safely change the user's config file?

Question

How to deal with a situation when I need to change config file structure?
For example, let's say I want to change it from this

{
  "device_address": "127.0.0.1:5001"
}

to this

{
  "device": {
    "ip": "127.0.0.1",
    "port": 5001
  }
}

If I'll do so, the new program release will fail to read it, which will cause the configuration will be lost, which is bad:(
But I can't stack with the same config structure forever - project is changing, requirements are changing...
Does anyone know any best practices for this issue?

Answer 1

I don't know if this qualifies as a "best practice", but one way is to do this migration in several steps:

1. First update the application's code so it can read either the old or new layout of the configuration file.
2. Ensure this new version of the application is deployed everywhere.
3. Finally, deploy the new configuration file.