json_serialize_deserialize.apex

Opportunity opp = [select Reason_for_No_Withdrawal_Symptom__c,Withdrawal_Symptoms__c from opportunity where id='006g000000GVeVq'];
Map<String,Object> deserializeMap = (Map<String,Object>)JSON.deserializeUntyped(JSON.serialize(opp));
Map<String,String> oppRecord = new Map<String,String>();
for(String key : deserializeMap.keySet()){
    if( 'attributes'.equalsIgnoreCase(key) )
        continue;
    Object value = deserializeMap.get(key);
    oppRecord.put(key,String.valueOf( value != null ? value : ''));
}

System.debug(oppRecord);
//--
Opportunity updateRecord = (Opportunity)JSON.deserialize(JSON.serialize(oppRecord),Opportunity.class);
updateRecord.Name = 'Test Record';
update updateRecord;
System.debug(updateRecord);