What changes for you
You get a modern web application without the quiet loss of features that rebuilds are known for. The fear is rarely the new technology. It is the behaviour nobody wrote down: the odd export, the rule only one customer relies on, the screen that looks unused until the day it disappears.
Your product owner and your support team know things no document holds, and their knowledge shapes what the new version must keep.
The old version stays live for your users while the new one takes shape. They do not notice a rebuild until you decide they should, and then they notice only that things work as before. Afterwards, changes are ordinary again, because the new version is built to be read, changed and handed on.
The new application lands in your own code repository. The code is yours. If you ever want to take it somewhere else, there is nothing to unpick.
What you decide
You decide when to switch. The new version replaces the old one when you are satisfied, and not before.
You decide what counts as worth keeping. Some differences are bugs the old application always had, and you may be glad to leave them behind. Others are features your best customers depend on. You make the call on each one.
You decide the order. The part that carries most of your revenue can come first, or the part that hurts your team every week. You can change the order as you learn more.
You decide whether to stop. You are never locked into finishing, and your code and your product stay yours to take anywhere.
What you see along the way
You see the new application running, early and often. You can click through the screens your users use every day and judge them with your own eyes. Your product owner and your support team can take part without reading a line of code.
You see changes arrive in your repository in small pieces, each with a plain explanation of what it does and why, so your engineers can read, question and approve them.
You see forgotten behaviours come to light while there is still time to deal with them. That is the reason to involve the people who know your product best.
Near the end, you decide the day. The switch is a decision you make with the result in front of you, not a leap you take on trust.
If the fear of what you will lose is what holds you back, start with one small area of the application and see how it feels before you commit to anything larger.
