LeeFJ I used FB for a long time.
Change IDE as any kind of software it's not an easy task.
First is the resistence to change and then the thecnical aspects of the project.
I moved all my projects from FB to IntelliJ about 2 years ago and very happy with my decision.
First, I installed IntelliJ with Flex module enabled on my machine and play around.
Then build a simple application with libraries.
I asked questions on IntelliJ support and a few of them originate an update on Flex module (since then IntelliJ base IDE evolved and the Flex module stopped evolved but for me it's ok. It's already good enought and there is no crash issues).
I tried to directly migrate a project from FB (hundreds of views and models) but end up forget it.
Migrate a project, software, db, there are always issues.
I end up creating a new project directly on IntelliJ, so it's a clean new project and copy/paste the src, locales and assets.
I had a few issues about the assets path (on my case was a mistake on FB that IntelliJ don't allow but FB let me compile). All this issues was identify on several migratain tests.
IntelliJ have a good refactoring engine (like any modern IDE) that help you out solving this issues.
After a week, I had 0 depedencies on FB and say goodbye to FB crashs becasue on memory issues.
Also, for my projects IntelliJ beats FB performance by far.
I have the very last version of their old perpetual licensing and I check every new version and it's always sand for the eyes. Nothing new that can improve the work for a Flex developer, so I will keep the same version and happy with that.
If there is any thing new on a future version, for me, then I will update.