I don’t believe it’s anymore difficult than transferring to any other school except you’d have to audition or submit a portfolio, I think. My friend’s suitemate just transferred from Gallatin to Tisch so it’s definitely not really hard. I really don’t know about the process but I checked the Tisch website and they said, “
How do I transfer from another division of NYU to Tisch?
If you are a student from another division of NYU (e.g., CAS, GSP, Stern, Gallatin, Steinhardt), you may complete the online internal transfer application here. Please note that some departments have portfolio requirements that must go directly to the department. Visit the department’s webpage for more about portfolio requirements. Students must be enrolled in the division to which they were originally admitted for one full year before they may transfer to another division.” (source)