[Zink Whistlefly]

From the Wiki:

The Holy Avenger (HA) weapon property include the following bonuses: +5 enchantment bonus (+5 to Attack Bonus and Damage) 1d6 divine damage vs. Evil +16 SR 25% chance of Dispel Magic on hit Only a paladin can gain the benefit of this property on a weapon. If the character has no Paladin levels this counts as a +2 enchantment. Notes A UMD score of 25+ will allow you to use the full benefits of the Holy Avenger as if you were a paladin. The dispel used by this weapon is more powerful than any dispel in the game and could be better described as "eat magic" rather than dispel. It can even dispel effects that no other dispel in the game can dispel such as petrification and fear auras from dragons. The holy avenger can completely strip a PC of all of their buffs with just one lucky hit. The 4th level paladin spell holy sword temporarily grants a weapon the holy avenger property The dispel effect will not trigger if you are not landing any physical points of damage on the opponent. For example, if someone had Epic Warding up and you are unable to damage them with any points of physical damage the dispel effect will never trigger. Characters who are not paladins and don't have sufficient UMD still benefit from the dispel OnHit.

As for it being upgraded, I have no doubts that it can be done. The base weapon enchantment will certainly increase, and as that passes the "Holy Avenger" +5 enchantment, it should override it. You should also be able to apply an elemental damage boost which should stack with the bonus 1D6 divine damage (only weapons which have damage boosts vs all creatures cannot have the 2D6 elemental damage applied).