Cancellation of an unregistered will

If all the heirs are at unanimity with respect to the will, they may make ideally recognise the identity of the  will and then execute a registered family settlement and supercede the same. Cancellation of any document is done on vitiating factors such as fraud/force etc and not by mutual consent. 

does making of registered family settlement involve a substantial amount of  money as stamp duty?

It is to be registered with whom ? -at any registering authority's office or where the properties are located?

If all of you are agreeable go for a settlement deed or partition deed.The document is to be registered with sub registrar of your local area where the property is situated.