Rewriting a major ecommerce site in parallel with maintaining the current one, while ensuring that all functionality stays exactly the same, that all cross-poll

Rewriting a major ecommerce site in parallel with maintaining the current one, while ensuring that all functionality stays exactly the same, that all cross-pollination between systems stays exactly the same... you make it sound like it would be easy. The stakes would be incredibly high, you would need extremely skilled, experienced and passionate engineers and a management environment that would give them enough breathing space to be successful. Everyone's careers would be on the line. Besides the fact that no one is going to want to stick their necks out for Target.com (whereas someone might consider it for a site that improves lives in a meaningful way), that's going to be a lot more difficult than you're making it sound.