I've replaced a few large legacy systems over the years and I disagree with most of the points mentioned in the link. The only way guaranteed to work is to run the old system concurrently with the new system with live inputs and compare results for a trial period and fixing discrepancies as they arise. Only when there is 100% behavioral/data fidelity with the old system can a switchover to the new system occur.

I'm not aware of the formal Design Pattern name, but a google search yielded this blog post on the subject: https://medium.com/@sahayneeta72/parallel-run-strategy-7ff64...

Coincidentally this is the same strategy github employed when verifying libgit2: https://github.blog/engineering/engineering-principles/move-...