Not only greenfield, but the problem domain is much better understood. A lot of architecture choices are made in the early days of a project when the problem isn't sufficiently understood to make the choice correctly.
I'm a huge fan of writing the first version of anything as an problem-exploration prototype, intended to be discarded and rewritten. As Fred Brooks said, "you're going to rewrite anyway, you might as well plan for it" [0]
[0] paraphrased from https://en.wikiquote.org/wiki/Fred_Brooks "The management question, therefore, is not whether to build a pilot system and throw it away. You will do that. […] Hence plan to throw one away; you will, anyhow."
In my experience, the prototype never gets thrown away when it should be, and sometimes it's never thrown away at all. It just gets extended, poorly, until development grinds to a halt because you can no longer add features or fix bugs without creating new bugs.
Then you either a) stop what you're doing and spend many months rewriting, or b) spin up a parallel team that does the rewrite, while the old team maintains the old code and does their best to add the most critical features and fix the most critical bugs without breaking anything else in the process.
Neither approach is good. (a) means you'll probably lose customers due to lack of progress on their pet issues. (b) means your development costs have doubled, and you have a team full of people who are demotivated and demoralized because they know they're working on something that's soon destined for the junk heap.
I usually build the first version expecting that it will live on for quite a long time (and sometimes/often be the only version), and build with an eye toward ease of refactor and even ease of rearchitecting. Yes, it's slower than building a prototype-quality product, and yes, sometimes product managers complain that the extra time needed will blow a market opportunity. Those PMs are usually wrong, and even if they are potentially right, building the prototype always takes longer than expected, so the PMs end up fretting over time-to-market anyway.
I'm a huge fan of writing the first version of anything as an problem-exploration prototype, intended to be discarded and rewritten. As Fred Brooks said, "you're going to rewrite anyway, you might as well plan for it" [0]
[0] paraphrased from https://en.wikiquote.org/wiki/Fred_Brooks "The management question, therefore, is not whether to build a pilot system and throw it away. You will do that. […] Hence plan to throw one away; you will, anyhow."