nlapiLoadRecord and nlobjrecord.setFieldValue

I'm writing a User Event script to update a field on the opportunity when a Sales Order is approved. I am utilizing the nlapiLoadRecord function and a nlobjrecord.setFieldValue method to set the field however it doesn't seem to be working. The values are stored within the variables properly but I have a feeling I shouldn't be using the Load Record function as Help only mentions being able to get field data from that record. Any suggestions would be appreciated.

function ue_oppty_rev_update(type){  /**      if (type != 'approve'){              return;          }  */      var so_rev = nlapiGetFieldValue('subtotal');      nlapiLogExecution('debug','SO Subtotal',so_rev);      var opp = nlapiGetFieldValue('opportunity');      var recOpp = nlapiLoadRecord('opportunity',opp);      nlapiLogExecution('debug','Opportunity ID',opp);      recOpp.setFieldValue('custbody_opp_so_rev', so_rev);  }