Opportunity Custom Fields
You can rename it, change its type, change whether it accepts multiple values
or not, change whether editing its values is restricted to specific Roles, or
update the options for a "choices" field type. The updated name will
immediately appear in the Close UI and only valid values for the updated
will be returned by the Contact API.
Some of the type changes need to convert all of the existing values for a given
Custom Field to the new type. When this is required, the response will include
converting_to_type field. When the conversion is done,
converting_to_type will no longer be returned.
The field will immediately disappear from any Opportunity API responses.