Labs/Leadership/How do you handle a lead developer's resistance to migrating their legacy system?

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How do you handle a lead developer's resistance to migrating their legacy system?

The technical migration plan is solid, but the person who built what you're replacing isn't on board.

Situation

You're leading a platform migration from a legacy embedded component to .NET 8 microservices. The technical strategy is sound: adapter patterns, dual-run validation, Strangler Fig extraction. But the lead developer who built the original system is showing resistance — finding edge cases, deprioritizing migration work, questioning readiness. Their deep knowledge is valuable, but their resistance could stall the project indefinitely.

Stakes

A failed migration wastes months of engineering effort and delays product roadmap
Silent data corruption from missed edge cases could impact customers
Team morale suffers if the resistant developer becomes adversarial

The lead developer just found another 'critical edge case' that supposedly proves the new system isn't ready. This is the third delay this month. How do you proceed?

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