Hi Sasikanth,
You have two different issues. I will try to address one of it.
I hope you are still testing in development system with the new numbering scheme. If yes, the reason the system is not allowing you to edit the numbering scheme is because you have assigned the tariff numbers from that scheme already to products and if those products are used on your test documents, then it stops you from editing the numbering scheme.
Here's what I would do to assess the situation further:
System Monitoring --> Technical checks-->Delete classification with new data (transaction /SAPSLL/PR_CL_DEL)
You have a simulation check through this transaction, please use it and identify how many products have got assignment with your relevant numbering scheme. You will get the relevant info to decide on your next course of action.
Please don't try it in Production System as it will create inconsistencies without testing it in development.
Hope it helps!
-Shrini