Hello,
I have done similar transitions from 32bit to 64bit for my own software and can do it for your application. I looked at your application, it is nice tool for musicians. But, before I can make final offer I need to know some details. Which language (languages) it is written? You noticed Objective C in your project skills list, but as your application is cross-platform, it can include some C/C++ parts besides platform-dependent UI code, or even some vectorized assembler code for data processing. Which of this is included? And how much code is in your project? Ideally I need to take a look at your code before starting project. I can sign NDA if you want.
My current bid is for case, when your code is not so huge in size, has reasonable amount of mathematics calculations (they are typically hardest to transition parts) written in Objective C/C/C++ and does not include assembly code at all.
I can start work immediately.
Regards,
Oleksandr Movchan