At a most BASIC level, most GameMaker Language (GML) code is the same between current GMS2 and older projects, but that doesn't mean older projects will just work... GMS2 changed some fundamental things in GML like how instances are created and also how a lot of graphics functions are referenced. That said, and project from GM:S 1.4 that is imported in GMS2 will be "auto-converted" to the new system and should run fine (and in my experience almost all my projects from GM:S1.4 worked fine after import). If you have projects from before GM:S 1.4, then those will not be able to be imported into GameMaker and you'll need to either:
1) Import them into GM:S 1.4, then export a GMZ then import that into GMS2
2) Recreate the project from scratch in GMS2 copy/pasting code.
Other changes with GMS2 that are more obvious and extreme are in the IDE. Drag and Drop has 100% changed so it's completely new and improved. The sprite editor was changed too, as was the room editor. There's also a lot of new resource types, like sequences, tilesets, animation curves. The resource tree is now an "asset browser"... and a lot of other things too!
You know that YYG offer a free 30 day trial on GMS2? You can download it and test it for yourself.