As others have said, I don’t think there is anything publicly available for this kind of thing. When we have had to make large-scale refactorings like this at LexiFi, we have sometimes written ad-hoc automatic tools to help us. It is not as hard as one would think 
The easiest case is when the transformation can be phrased in purely syntactic terms. In this case, you can write a tool using compiler-libs that parses each file, obtaining the corresponding Parsetree. Then you perform two passes. First, then you walk through the Parsetree, keeping track of the the places where you need to change something: in you situation, it would be the match scrutinee and the final expression of each match case. Second, you do a “rewriting” pass where you use the information that you obtained in the previous pass to textually insert the new code fragment in the original source files.
If the transformation cannot be phrased in purely syntactic terms, then you need to combine it with reading the .cmt files which contain the type-annotated Typedtree in order to find the places that need rewriting, and it is a bit more involved, but the overall logic remains the same.
Of course, it is so easy to do this kind of refactoring “by hand” just by following the compiler errors that the investment of writing this kind of tools is only worth it if you have a large codebase to refactor. It was worth it for us at LexiFi (~600k LOC), but for more reasonably-sized codebases the tradeoff may be less clear.
Cheers,
Nicolas