Hi Sneha,
I have not explored the SAP LT to that extent.
But for your query, I think it will not touch the already replicated data. It might not even allow to apply the type transformation as tables already have data. You might have to restart the replication for tables having date. But really not sure on this. There is a separate community totally for SAP LT in SCN. You can raise this query here http://scn.sap.com/community/replication-server.
In our scenario we are keeping the date fields as NVARCHAR(8) and left it to that. In case of any calculations based on dates, we have used time functions and/or Time Attribute Views in HANA. If you want to go for a similar logic, I might be able to help further.
--Shreepad