Serial Number For Existing Stock 

Generation of serial numbers for existing stock in storage location.
This is regarding the serial number profile and equipment generation.
For the future transactions if the serial number profile is activated then the equipment generation will be done.
But for the existing stocks available in Storage location, how should we manage this activity i.e. how should we generate serial numbers. 
As per our knowledge we need to move material out to a dummy material and then maintain serial number profile for the reqd material, then take back the stocks again. But, as we need to do this activity for huge number of materials, so please suggest if there is any alternative, so that we can do this activity faster.
There is no alternative for this, Please do not make the mistake of activating serial number profile for a material that has stock, we have faced similar issues in the past.
Best way to remove the stock, activate serial number profile, take back the stock in with serial number.
Use 201 & 202 movements for this as you can track down any discrepancies or issues.
Serial Number change for existing stock.
We have stock of many materials for which we have serial number assign to it, now we need to change the serial number of existing stock, like we have One material Code ABC and stock is 100 sr. no. is 1,2,3, etc. now we to change this as 09111001 to 09111100. How to do this and how to assign default sr. no. in format 09111001 onward means YEAR - MONTH - LOT NO. - Sr. No 09 11 1 001
Once assigned serial number is not possible to change.  You need to reverse the goods movement and do it again. 
You cannot direct change the serial numbers for the existing stocks.
For that what you can do is:
Cancel all the Inventory documents against which you have received that serial number stock.
After cancelling the documents, then create the serial number in IQ03 Tcode.
Now while doing GRN for all the above stock which you have cancelled above, mention the serial numbers as 09111001 TO 09111100 .
This will solve your problem.

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