Phase 1: Core Implementation (TDD)
├── Add @deprecated markers to old code
├── Implement new algorithm with tests first
└── Target 80%+ coverage

Phase 2: Data Model Migration
├── Extend existing models (don't replace)
├── Implement MigrationStrategy enum
└── Add version detection to parsers

Phase 3: Service Layer Integration
├── Create git tag BEFORE changes
├── Update services to use new algorithm
└── Integration tests

Phase 4: Cleanup & Verification
├── Create final git tag (restoration point)
├── Remove deprecated code
└── Full test suite (500+ tests)

@available(*, deprecated, renamed: "NewAlgorithm", message: """
    OldAlgorithm is deprecated and will be removed in v2.0.
    Migrate to NewAlgorithm for improved accuracy.
    """)
public enum OldAlgorithm { ... }

public enum MigrationStrategy {
    static func migrate(oldRecord: OldFormat) -> NewFormat {
        NewFormat(
            // Preserve critical fields
            id: oldRecord.id,
            lastModified: oldRecord.lastModified,
            // Transform algorithm-specific fields
            newField: convertOldToNew(oldRecord.oldField)
        )
    }
}

public static func detectVersion(_ data: String) -> DataVersion {
    if data.contains("new_field_header") { return .v2 }
    return .v1
}

# Before cleanup
git tag v1.x-pre-cleanup

# Emergency restoration
git show v1.x-pre-cleanup:path/to/OldAlgorithm.swift > OldAlgorithm.swift