Internal transfer to Stern from Undecided Freshman CAS

I was accepted in Undecided at NYU CAS in 2018. I deferred it for a gap year to until 2019 fall. During this time I figured out I want to cross school double major in Business in Stern and Politics in CAS. I read that you are only allowed to do this cross school major if you are in Stern but not CAS. Does that mean I have to first transfer to Stern? If so, can I apply to stern before starting freshman? Is there anyway I can be in stern in my freshman year? Does undecided give me a better chance to get into stern CAS?

I just wanna do my cross school double major in politics at CAS and business in Stern. why is it so confusing and what do i need ti dooo to get this?
Thank you guys so much, super confused!