Managing complexity in large software projects is a significant challenge that requires a multifaceted approach. And I would like to know how you manage this? what are your priorities in this management? how do you look for qualified personnel to assist you? since often the project is not even funded. Let us know in the comments how you're doing or have done if you've already experienced this.
have it all planned out on paper. If you fully know what will need to be done in your roadmap of the project you will be able to change and add things to your plan later down the road while assuring compatibility. It sucks to get started on a project, then later the enxt thing you need to implement dont work with your previous code. If you planned it all out before on a piece of paper so you can visually see what you need to do, you would be able to make sure that all the dependencies for your future implementations wont interfere with each other.
but there are times when you have to change a lot of things because the audience you want to reach asks for it and you see the need to do so. as happened here with this website, at the beginning the idea of the post feature was not paramount but now, and with the requests of our users, we are already implementing new things to this feature.