New York State law.
Is it legal for someone to voluntarily assign their benefits from a will to a family member ? Would a letter of assignment be the proper way to do this ?

Yes its legal. If you are listed as a beneficiary of a will to receive specific property (i.e. a house) you can assign your rights to that house to a third party. That third party would then resent the assignment contract to the executor upon the probate of the will.

However, the testator can change their will at any point leaving the assignment worthless.

Is there an issue in doing that after the testator (thanks for the new word ) is deceased ?Can the executor make the assignment ?

After the testator is deceased but before the estate has been distributed and processed it amounts to the same thing. However, once the estate has been distributed then the estate no longer plays a part.

What we are really talking about is a sale. Depending on the nature of the property, there may be other implications.

No this is not a sale. The beneficiary is likely to lose all the assets if the person applies to Medicaid . So the beneficiary would prefer that it remain in the family .

See, this is why it's a good idea to explain the situation rather than just ask an abstract question.

Medicare can go back at least three years (may be longer now) to access assets. So if the beneficiary of the will applies for medicare within three years of the assignment it will be nullified.

And this becomes either a sale or a gift. The beneficiary of the will has a legal right to the property upon the death of the testator. If the property is transferred to anyone else, either before or after the beneficiary takes ownership, then that transfer is either a sale or a gift and there are tax and other consequences to each type of transaction.

So, if the benenficiary wants the property to remain in the family, then the family has to purchase the property from the beneficiary for fair market value. The proceeds of the sale can then be turned over to Medicare.

Thanks Scott ,that is what I suspected . I cannot rate your replies but this has been very helpful .

One more question if you don't mind.

If beneficiary would be able to live without utilizing Medicaid benefits through the 3 years (or longer ) ;could the beneficiary transfer the assets to the family prior to the time that Medicaid peruses the applicants assets without negative implications ?

The transfer would have to take place three years prior to the application for Medicare benenfit.

Again, I'm not sure what the actually time frame is now, it may have changed. The only way to get around that is if the medicare applicant receives fair market value for the asset which is then part of the Medicare calc. Medicare would actually prefer cash assets. So they would not be adverse to letting the applicant sell property to "keep it in the family". As long as its for fair market value.