Important differences between 4.3.1 (stable) and 5.0-dev?

Currently I am using 4.3.1 stable. And based on some Issue comments I know that some of the problems with 4.3.1 are fixed in the development snapshot.
But I can only found a 5.* version. This sounds far away from the current stable release.

So before testing this and dumping my bib-file again (like it was between 3.8.2 and 4.0) I want to ask if there are any important Issuses between this version the users should know?
Maybe is there a transformation of the bib-file format again?
Can I step back to the stable version if I want?

I cannot remember that we changed anything with respect to the file format. You are nonetheless advised to make a backup before trying out the new version.

The changes in the dev 5 version concern mainly the user interface. The main entry table was completely rewritten and many dialogs converted to the new UI technology. Hence the jump to version 5. There are still some bugs that needs to be fixed before we can release a beta version but early feedback is very welcome!

If you exactly want to know what changed:
shows all commits since 4.3.1 (over 500, showing only the latest 250)

Mostly related to the conversion of JavaFX and fixes in the interaction between old and new gui.