Bava Batra 126 - Changing Allocations in the Inheritance
If one says, "So-and-so, my firstborn son, shall not take a double portion of my estate," or "So-and-so, my son, shall not inherit with his brothers," - he has said nothing, that is, his directives are not followed, because he has made a stipulation contrary to what is written in the Torah.
However, if one distributes his property before his death, increasing the share of one and decreasing the share of the other, or making the portion of the firstborn equal to those of the other brothers - his words stand, because he conveyed the portions of his estate as gifts rather than bequests.
If at any point in a document, whether at the beginning, the middle, or the end, one wrote that the awards are conveyed as "gifts," even though the document also contains language of inheritance, his words stand.
Art: William McTaggart - Father is at the Helm, 1889