Migration strategy 2.5 to 3.0 Docker (IOT Stack)

I am planning to upgrade from 2.5 to 3.0, and and I want to have the system down as short time as possible. I have a production system and a test system
I have a lot of MQTT items with 1.x Binding, that will take long time to convert Upgrade from 2.5 to 3.0, using MQTT 1.x binding - Setup, Configuration and Use / Migration - openHAB Community

What is the best strategy for upgrading? Start from scratch in a test system and when that seems to be working transfer it to the production system (Copy my config files to the production system and get the 3.0 Docker).

Or is it better to copy all the config files from the production system to the development system and thereafter upgrade it to 3.0 and start with converting all the MQTT items and when that is complete copy the conigs to the prouction system and get the 3.0 docker?

Or are there any better approaches?

Just another approach, since you are using docker. Set up OH3 with a different set of data volumes & different port mappings.
Use the remoteopenhab binding to access the OH2 container. You can them map the OH2 Items to OH3 Items and work on migrating your devices to the MQTT v3 binding as well as use OH3 rules & UI.

In my experience, use the server ip address & mapping to access the OH2 server even though it is auto discovered on the Docker network. That ip address changes after a recboot.