Dear Mr Mithbavkar,
Let me state here first what I could understood of your query
I think your question is you have two units, one is running and registered under central excise as well as in service tax, the second unit is closed since long, now you want to let out that 2nd unit , if I am right here is your answer:
Please check status of the second unit which is closed since long, whether that unit is having a self entity or that unit has been merged with this one which is running, what I mean is who owns the unit which is closed , if that unit is owned by the unit which is running and as per all statutory records this has been declared, yo can carry on with the S T registration that you may be having for the unit which is running, you just need an amendment to that but if the above is not the situation you should get a fresh registration in the name of the second unit or its owner as the case may be.
Hope answered your query
With regards,
Neeraj