I hope (and I believe I echo Borror0's thoughts on this) that the Holy Avenger (HA) will not give different abilities to different levels of paladin. If it did that it would be a pseudo enhancement line for paladins and every paladin would have to own one. It would in essence be a crutch to bolster the paladin class, rather than purely a weapon choice.
I believe that the HA will have abilities that will scale with the more paladin levels you have such as the expected Spell Resistance of 5 + paladin level. I would also hope that they would include a couple of fresh ideas as enhancements to it such as giving it an improved turning type of ability for other paladin abilities. (e.g. For every five levels of the paladin class that the wielder possesses this weapon increases the effective level for determining the power of your <class> ability by 1. <Lay on hands / Smite / Caster Level / Divine Grace / etc>