Pre-Mortem Probate

Pre-mortem probate surely that's an oxymoron? A will becomes effective only upon the death of its maker. To probate a person's last will and testament, meaning to prove the will's validity, months or even years before the person dies seems problematic, to say the least. What if the testator makes another will later? If the court finds a will invalid pre-mortem, are later codicils invalid as well?