Breaking changes for Godot 3.0?

Question

Is there a list of breaking or potentially breaking changes for porting projects from Godot 2 to 3? Or is it definitely backwards compatible? Just thought I'd ask.

Answer 1

There are indeed some breaking changes in Godot 3 that will prevent compatibility with 2.x but that is somewhat inevitable with a completely new renderer and some internal changes in the API. A lot of these changes are explained here. You might also find some information about the changes in this Google Drive for the Godot 3 breakage.
However, there already is a WIP version of a Godot 3.0 exporter available in Version 2.1.3 right now which is going to be improved and completed for 2.1.4. It will make the porting process as easy as it can get but, depending on the range of your project, you might need to do some manual work in order to complete the process. The rest should not be to much of a problem though. There will also be documentation available for this.
I hope that cleared some things up, if you have any more questions feel free to ask!
Have a nice day :)

Answer 2

Not backwards compatible, many classes changed internal things and all have access to some properties (apart from the getters).

Some like Transform changed a lot too, Physics engine is getting some fixes and materials are totally different, also shaders and particles.

While it will be possible to use the 2.x to 3 exporter I think that, for big projects, will be a lot less work to migrate scene by scene, script by script.