When you have two record, one being the existing queried record and the other the updated record. To merge the original with the updated field without committing the changes you can iterate the current record’s fields and check which of the fields changed. Update the changed fields to the new updated fields value. Now we have an merged record. Here is how I went about doing it.

Original SObject record

{"Account_Type__c":"Managed","Amount__c":50.00,"Category__c":"Deposit",
"Description__c":"Deposit","End_Date__c":"2020-03-31T15:06:57.000+0000",
"Frequency__c":"Monthly"}

Update SObject record

{"Amount__c":500.0,"Id":"a0L63000000qc43EAA","Frequency__c":"Annually"}

Merge two records with changes

private static Set<String> fieldsToExclude = new Set<String>{'OwnerId','OtherGeocodeAccuracy','MailingGeocodeAccuracy','BillingGeocodeAccuracy','ShippingGeocodeAccuracy'};
public SObject mergeRecords(SObject updatedRecord, SObject currentRecord, String sObjectName){
  SObjectType objToken = Schema.getGlobalDescribe().get(sObjectName);
  DescribeSObjectResult objDef = objToken.getDescribe();
  Map<String, SObjectField> fields = objDef.fields.getMap();
  Type classType = Type.forName(sObjectName);
  SObject mergedRecord = (SObject)JSON.deserialize('{}', classType);
  for (String field : fields.keySet()){
    if (!fieldsToExclude.contains(fields.get(field).getDescribe().getName()) && (fields.get(field).getDescribe().isUpdateable() || fields.get(field).getDescribe().isCreateable())){
        if (updatedRecord.get(field)!=null){
        	mergedRecord.put(field, updatedRecord.get(field));
    	} else if (currentRecord.get(field)!=null){
        	mergedRecord.put(field, currentRecord.get(field));
    	}
    }
  }
  return mergedRecord;
}

Test that fields were merged correctly

@isTest static void testMergeRecords(){
  String currentRecordJSON = '{"Id":"a0L63000000qc43EAA","Account_Type__c":"Managed","Amount__c":50.00,"Category__c":"Deposit","Description__c":"Deposit","End_Date__c":"2020-03-31T15:06:57.000+0000","Frequency__c":"Monthly"}';
  String updatedRecordJSON = '{"Amount__c":500.0,"Id":"a0L63000000qc43EAA","Frequency__c":"Annually"}';
  Type classType = Type.forName('Account');
  SObject currentRecord = (SObject)JSON.deserialize(currentRecordJSON, classType);
  SObject updatedRecord = (SObject)JSON.deserialize(updatedRecordJSON, classType);
  Test.startTest();
    SObject mergedRecord = mergeRecords(updatedRecord, currentRecord, 'Account');
    System.assertEquals(mergedRecord.get('Amount__c'), 500.0);
    System.assertEquals(mergedRecord.get('Frequency__c'), 'Annually');
    System.assertEquals(mergedRecord.get('Account_Type__c'), 'Managed');
    System.assertEquals(mergedRecord.get('Category__c'), 'Deposit');
  Test.stopTest();
}