Update a custom field scoped to a contract type. The appliesTo and contractTypeId values cannot be changed through this endpoint.

fieldType can be changed only when every non-empty stored value is already valid for the new type
(values are never converted); any type change is allowed on a field with no values. A currency field
can't change type while it is calculated or while a calculated field is based on it. A refused change
fails with 422; typeChange reports how many values fail, out of how many, plus a sample of contracts
and values to fix or clear first — the same check as PUT /customFields/{id}.