Client API JS call does not trigger on OnChange

Hi

I've written the following to update fields on the opportunity with data from a related entity. This should happen on OnChange, but the field are not populated before the form has been saved

Can you see what is wrong?

function retrieveServiceAvailabilityInfo(executionContext)
{

var city;
var postalCode; 
var province;
var serviceArea;
//var policeGuard;

formContext = executionContext.getFormContext();
var serviceAvailabilityEntity = formContext.getAttribute('cgi_serviceavailability');
var sAEID = 'F0A708D3-9700-EA11-A811-000D3AF3A041'; //serviceAvailabilityEntity.getValue()[0].id;

Xrm.WebApi.retrieveRecord('cgi_serviceavailability', sAEID,'?$select=cgi_city,cgi_postalcode,cgi_province,cgi_servicearea').then(
function success(result) 
{
city = result.cgi_city; 
postalCode = result.cgi_postalcode;
province = result.cgi_province;
serviceArea = result.cgi_servicearea;
//policeGuard = result.cgi_policeguard;

var data =
{
'cgi_city': city,'cgi_postalcode': postalCode,'cgi_province': province,'cgi_servicearea': serviceArea
}
Xrm.WebApi.updateRecord('opportunity',formContext.data.entity.getId(), data).then(successCallback, errorCallback);
formContext.data.entity.save(); 
},
function (error) 
{ 
alert('Error');
}
);

}