The double challenge here is doing this all whilst essentially keeping it in the background out of any customer sight. Even if you know exactly what you need after a while from a business perspective, you still need to reimplement it in a way that doesn't cause your product / service / platform to lose customers. I find this always to be an extreme challenge. It's a bit of a treadmill too: doing it this way (without causing breaking changes) certainly takes longer too. So it all piles up into a big messy stack of work :)