Migrating to new scalar type in backwards compatible way

I have run into a tricky issue where we have been using the built-in Int GraphQL type for a field on our schema and its come to our attention that we have some use cases where we know that field will be larger than a 32-bit integer. We are planning to introduce a new scalar type for these fields such as Decimal or other but are running into issues on how to do this in a backwards compatible way. We first looked into using a union IntOrDecimal = Int | Decimal but found the spec does not allow unions to be used in input objects for our mutations. Is our only option to create new fields and migrate all clients? We have a mobile app so that means we would perpetually need to support both fields for quite some time. Or does anyone know how to override the default validation on the built-in int type in the graphql-js library?