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Original strangler fig post is here: https://martinfowler.com/bliki/StranglerFigApplication.html

I second this

> Follow the strangler fig pattern, and map out every single task that is required in the migration on a whiteboard.

> Write tests if you can, and set up a staging environment for V2 that you can setup and tear down easily for battle testing way before going live.

I've successfully helped migrate a critical project and followed exactly this strategy. Older versions were being developed and run 1:1 in parallel to the newer ones until the customers got only a small downtime due the change of IP Addresses were the system was running

This is a good answer and one I've put into practice successfully more than once. Automated tests are very key here.