Executor mentioned changes he wanted to do in will

Indeed, the entries made in your grandfather's diary, (if he maintained the diary on a regular, personal basis, ) would be admissible under section 32 of the Indian Evidence Act.

s32 essentially lays down that statements of a person are relevant if that person is dead/cannot be found. Read with subsection 2 and 6, you could very well challenge the previous will.

To clarify again, 32(2) lays down when the statement was made "in course of business. —When the statement was made by such person in the ordinary course of business, and in particular when it consists of any entry or memorandum made by him in books kept in the ordinary course of business, or in the discharge of professional duty; or of an acknowledgment written or signed by him of the receipt of money, goods, securities or property of any kind; or of a document used in commerce written or signed by him; or of the date of a letter or other document usually dated, written or signed by him.

(6) or is made in will or deed relating to family affairs. —When the statement relates to the existence of any relationship 25 [by blood, marriage or adoption] between persons deceased, and is made in any will or deed relating to the affairs of the family to which any such deceased person belonged, or in any family pedigree, or upon any tombstone, family portrait, or other thing on which such statements are usually made, and when such statement was made before the question in dispute was raised."

Such statements would be relevant, even if the sons aren't aware of the changes made in the will.

Good luck!