Can a will be changed once during the lifetime of the testator?

Look, for giving proper suggession in this regard of Hindu Joint Family a big discussion will be needed. Otherwise, no Advocate will give you proper guidence regarding it.
Because, as per your version, the Property are Joint Hindu Family property and you are belongs to Mitkhara family of law.
Therefore, please met with an Advocate who have enough knowledge regarding Mitkshara Family can give you proper suggesion.

A will can be changed as many times as he wishes during his lifetime . There is no restriction.Your grandfather may make a fresh will declaring equal share of his 5 sons and he may declares there his previous will as cancelled. It would be better in future make the will registered.

The last will is what matters and as such the last executed will of a person would be effective.
Actually the most important part is that the previous will is not required to be revoked or cancelled and the same would be revoked automatically.

Your grandfather can change his will as per his wish and he is making another one and he should mention in the second will that the first will stands revoked and this present will stands into force and mentions his division of property.

Sir, your grandfather can change his will as many times as he wants by making codicils or by making a will/s afresh, during his lifetime. If he does not make any will, or if such will is not probated from a competent court, the property of your grandfather, after his death, will be equally divided amongst his wife and all sons and all daughters. It does not matter in whose name the property bill and electric bill stands. please contact for more details.