Are you using 3rd party C libraries in your Mac app? Or maybe some custom assembly code? Then things will be different when you switch to 64-bit macOS.
Maybe this post will give you a head start. It shows how you can use static libraries now, and how you can use an external assembler for your assembly code.
https://blog.grijjy.com/2019/07/19/using-static-libraries-and-assembly-with-64-bit-macos/