Can a spa be legal if made over an existing spa?

first of all,  you can cancel the SPA on stamp worth Rs.20/- and issue registered (with acknowledgement due)   notice to the person to whom you have given power of attorney that you have cancelled the SPA., though it is un-registered.   then you may give power to your son.  this is for information only.

 u CAN CANCEL  THE  SPA by giving Public Notice in two to three Leading news papers

and by addressing termination notice

You can also Appoint your son as your attorney and register it with the concern sub-registrar of assurance

and in that power in recitals you can give reference of termination of the earlier SPA

and also enclosed the Termination Notice and Public Notice Xerox Copies so it will become a Public Document.

And it is presumption in law that any document which is registered the general public is having deemed knowledge

from th edtae of registration