Transact SQL :: Inserting Distinct Data From One Table To Another

I have 2 tables: Source Table - IncidentDimNew and Destination Table - IncidentDimNew with identical columns: Id and CreatedDate but the data is different.

I would like to insert distinct data from those 2 columns of 1st table into the same 2 columns into the 2nd table but I onlt want to replace the values in the 2nd table where the Created Date is > 2015-04

I compiled the code in order to get the information I want from 2 tables:

Source Table
SELECT
COUNT(Id),
LEFT(CONVERT(VARCHAR(10)), CreatedDate, 126), 7)
FROM
IncidentDimNew

[Code] ...

This is the code I wrote in order to do it
                     
INSERT INTO IncidentDim 
[Id]
      ,[CreatedDate]
     SELECT
[Id]
,[CreatedDate]

FROM
IncidentDimNew

where left(CONVERT(VARCHAR(10), CreatedDate, 126),7) > '2015-04'

GO

But what it does it's gives the SUM of the values in the corresponding rows where the values are not null which I don't want.

So  how do I leave the values as is from table IncidentDim and insert new values as it is from IncidentDimNew where the Created Date is > 2015-04?