Hi Kumar,
For your requirement can you please check if you can make use of the Movement Type 309 E (TCode MB1B). Using this you can create a transfer posting of the Sales Order Material 300000-01 to the new material 300000-03. The below mentioned link will give you a basic idea of the steps to be followed (Source - help.sap.com):
http://help.sap.com/saphelp_afs50/helpdata/en/72/31b489855e11d4a6970060086fd66b/content.htm
By this method you should be able to meet your requirement of converting the inventory of the old product to the new product. Kindly explore this idea further as to whether and how it can be applied to your busniess scenario and inform us of the result.
Regards,
Som
