Yes, and not just for backcompat reasons - as recently mentioned in #[repr(transparent)] – Why? wrapping something in a structure can change how it's treated in ABI. For Rc<T> to be soundly transmuted to Rc<U>, Rc<_> itself would need to have an appropriate repr, which it does not.
from_raw and into_raw are indeed the exact mechanism by which you should do this, and they work reliably in this case because RcBox<_> is repr(C).