The most important thing when rewriting the old code is to decide which framework chose Yes framework or maybe only some utility classes ex smartypants ioc is better . I will say go with IOC 100% to separate your view from the logic part. You can loose some time when you've got some not organized code, or you are using the oldie framework but it will benefit in the future.
First Separate your business logic.
Badly written project have some business logic mixed up with the view logic. Services with specific calculations. When you will try to rewrite your application try to group your business methods in one part and the view logic in the other part. It's easy in the Robotlegs Mediator so you can see what methods have it's view.someFancyComponent reference and what have not. Put the minimum in the view components logic and separate other things to some Helper classes that you can refactor later. If your project is quite big and getting larger you will see you have some helpers that methods are the same but maybe some little part is different. Try to use it as params. Once you clear your view logic out from business logic you can separate the business itself to services logic and equations logic. Do equations in separate classes call it some strategy. ex. MyBudgetImportantStrategy if you are using DI you can insert it and decouple with interface and you can use unit tests in the future. The equations logic merely changes when it's done right. And you are ready for webworkers dude. You can then put your all equations to the worker. The update will be 1-2days not 1-2weeks, months when you wrote your code wrong and keep doing it.
Second be elastic with gui.
Well the gui part is the most important thing in the flex. It have changes. Well it is changing all the time. Maybe you remember some css issues when migrating from flex 2 to flex 3 and you will remember rewrite to the flex 4 and then you will remember if your boss told you compile it to mobile. The simple part is IOC. When you have IOC framework with your business logic highly separated from the gui. The gui logic is the bottleneck. If you screw this up and you need to refresh the look of your big enterprise application you are in hell now. You need to keep your mediators smart and simple, decouple the state and change layout logic from the interaction logic. The layout logic it the shit when you need to reorganize your components view the all the old layout is often the shit see the flex 3 and flex 4 states, flex 3 and flex 4 css. But the button clicks as it clicks it is the main functionality for it, combobox often is only combobox and list is a list. But it may be in the different state of your layout now. When you put your view components first think what is your functionality for this control. And try to put the functionality components separate from your layout component's don't mix them up or they will mix you up when refreshing the look.
Think, read and ask, there are many people out there that have the same problems and there are many authorities in the world of flash/flex that write some good shit about every problem you have. But don't chose the first solution of your problem the person who wrote it 3 years ago is not the same programmer now, maybe someone did it better so you can use it . Try to read more, yes it's time consuming but when you rewriting you don't want to rewrite it again and again. Maybe sometimes it's better to mix some solutions up to build something better. Use event's as much as you can you can. Man if you have some performance problems why then not use signals or do some own simple notifications where you send often. The event's are for you to keep your life easier not to kill you. Do the commands. The command is the shit cause it only have one functionality. You can separate it and use it in your logic.
Prototype only when your time is very limited but try to separate the 4 logical parts
- layout, - interaction,- services, - equations
cause flashplayer is crossplatform only from the basis and you interact with phones, tablets other then on desktop or even game console. Your layout and functionality may be different for other screens and equations could be moved to the server when low performance. And on there is still more freedom on the desktop.
Happy codding ;)