You can use a macro for your structs to make them changeable while prototyping. I think it is called @prototype or soemthing. You need a package for it. Google it.
and ProtoStructs: https://github.com/BeastyBlacksmith/ProtoStructs.jl
I’ve never tried them, but they’re advertised as doing the job.
Addendum for those who are new to this, the idea is to use these macros during development, and then get rid of them once the types stabilize (since they tend to have a performance cost).