It will only exclude the specific property gifted from the distribution stated in the will. There is no need to redraft the will. Except the property gifted the terms of will remain intact.

There is no validity for a will. It's for perpetual time. Only for probate it's 3 yrs from death of the testator after your knowledge

You are at liberty to execute gift deed during your lifetime 

it would not invalidate entire will 

The Will shall come into force only after the death of the testator hence the transfer can legally take place only after the death of the testator and not before death, hence the Will can be considered as ineffective during the lifetime of the testator.

Therefore the the transfer of property by a gift deed shall be considered as the legally valid transaction, so the beneficiary of the Will in respect of the said property  cannot dispute this transaction which had taken place during the lifetime of the testator, while enforcing the Will. 

The Will shall be valid in respect of the other properties which ae still enforceable as per Will.

A Will is intended to dispose off property. There must be some property which is being given to others after the death of the testator.

A Will becomes enforceable only after the death of the testator. It gives absolutely no rights to the legatee (the person who inherits) until the death of the testator . It has no effect during the lifetime of the testator.

- As per law, registration of a WILL is not mandatory , and it can be written on a blank paper only in the presence of the two witnesses. 

- Further, a WILL can be write many times during life time , and only the last WILL is valid 

- Since, you have already mentioned that property in the WILL , then if you will execute a gift deed in favour of that son , then others share will be intact as mentioned in the WILL , and you will have not to write a new WILL. 

Gift of one of the properties shall not invalidate the Will. That property alone shall stand withdrawn from the bequest later and the Will shall be valid for the remainder of the legacy.

Will comes in existence after the death of the executant.  You can deal with your properties in whatever manner you like during your  lifetime. After your death whatever properties are not dealt with by you would be governed by existing Will.  

Secondly, you may change your Will anytime or any number of time in your lifetime and in that case last Will would prevail. 

Will is effective after the demise of testator. Gifting the property does not invalidate the WILL. To make it more simple, execute codicil to the WILL.

If you are gifting a property which is the subject matter of your registered will by way of a gift deed, then it is advised that you execute a fresh will wherein also mention the gift deed in order to avoid any disputes in future amongst the children.

No, you can change anything in you will as many time you want during your lifetime fully or partially. Latest would be considered final.

You can cancel if fully or add something in it or reverse any term of it by making codicil / additional will.