Warning: If you send fieldOptions, it is a full replacement of the
options. Every existing option whose numeric ID you leave out is permanently
deleted, and [] removes all options. To keep the options unchanged, omit
fieldOptions or send null. Options with a null or unknown ID are created.

displayLabel is required; fieldName and fieldKey cannot change. Include
the existing objectTypes: if you omit it, it defaults to [] and clears the
scope. Other nullable properties keep their values when null or omitted; you
can set empty tooltipDescription or defaultValue strings.

You can change fieldType even when the field is in use, but existing values
may become invalid. This update does not check the URL default as create
does. Requires the CustomField:Update permission.