You'll find that the degree of automation introduced to the migration process will vary widely depending upon whom you hire to perform the work. You should probably treat claims that completely automatic conversion tools are %100 successful by maintaining a healthy degree of skepticism. The separation/incommensurability referred to within the first reply leads to such software almost always being bundled with professional services and rarely, if ever, offered for sale as a standalone product. Design decisions/judgment calls are required far too frequently to trust the outcome to software (although the degree to which manual intervention is required shrinks daily).
(do note that, on some level, the paradigmatic nature of the difference between various manufacturers remains somewhat tempered by the fundamental goals & design of the communications protocols their products are supposed to be designed around.)
Redesigning your entire infrastructure is not always necessary, but the more flexibility you bring to the cutover process, the better your results tend to be.
A relatively safe generalization concerning conversion tools might run something like: "writing your own firewall migration automation tools provides insight into conversion issues difficult to otherwise come by/stumble upon."