Even if there is a registered Will, it needs to be proved ad the last Will of your GF by having it probated
Mere registering the Will does not dispense with its proof
So all the legal heirs of the GF will have to give their consent affidavit stating that the Will is true and genuine Will of your GF and they have no objection if property named in the Will is transferred to your father's name who is a legatee in the Will
Even for mutating name in 7/12 the NOC of legal heirs would have been taken by the revenue officer
However 7/12 is not an ownership document. It does not confer any title on the person whose name reflects in 7/12. It's only for revenue purposes and nothing else
The property which is bequeathed under the Will will be required to be transferred to the legatee by registering a transfer deed in his name. That confers ownership right on the legatee
That can happen only if the Will is probated
Only thereafter can the name of the legatee be entered in property card records
Without a registered title document in your father's name, after his demise, his legal heirs who are you and your mother will find it difficult to either sell the property or enter your names in property card
Thus proving the Will, despite it being registered, by applying for probate is essential