Studio is a bit more "grown up" and is more strict about some things. For instance, resource names are now
enforced to be OK to use in code, so you can't use spaces and symbols like < in them anymore. You can't use sleep(), screen_redraw() or screen_refresh(), and execute_string() and the variable_local_exists() family has also been removed. It's also worth noting that resource IDs get reassigned each time you compile your game, which can lead to issues with things like custom level editors. Instance Creation Code is now run
after the Create event, which actually streamlines custom variable assignment (you don't need to check if a variable has been assigned in the create event before you give it the default value). Long story short, if you've been using some of the advanced features, chances are your game might not import properly, but feel free to pop by the Programming sub-forum for suggestions how to fix that. (For instance, you can get around
execute_string() pretty easily by using
execute_script() instead and make one script per hardcoded string you'd execute)
There's also a huge amount of GUI improvements - for instance, you can zoom in the room editor, which makes editing large levels a lot easier, especially with the new 'big picture' view you get when you zoom out a bit. You can highlight instances with creation code in the room editor, and also hide invisible instances making it possible to 'preview' the room. You can also stretch and rotate objects directly in the room editor, which can be used for all-new ways to design levels. The sprite editor now has full transparency support (I don't even remember how it was to have a transparent color and on/off transparency!) and some new tools. (Most of these improvements actually were GM8 stuff, but they've carried over to GMS and been further refined). There's also a large amount of new functions, but I've ranted enough about this
It's a huge step to GMS, but even if your current projects can't be converted, I
strongly recommend switching once you've finished them - GMS has a bunch of pretty great workflow improvements that you'll enjoy.