"Whenever you attack with the bonded weapon, you can use your Charisma modifier for the attack and damage rolls instead of using Strength or Dexterity; and you can cause the weapon to deal Necrotic, Psychic, or Radiant damage or its normal damage type.
If you are asking what I would do at my table, the answer is change it as desired and change all the damage on the attack (so extra cantrip damage, extra weapon damage, Hex damage etc). That just seems the easiest and most straight forward.
As far as RAW, it really rides on the definition of "normal". IMO Pact of Blade as it applies to non-magic weapons actually creates the weapon out of thin air. The "normal" damage is whatever the non-magic weapon has.
When applied to a magic weapon I would say everything that magic weapon does as "normal" for that weapon. In this respect it would let you change all the damage, but it would be all or none. If you have a dagger of venom you have envenomed and change to Radiant damage then both the slashing and the poison change to radiant.
I would also say this would need to be a property of the weapon specifically. So if you are using Hex the Hex damage does not change. Similarly if you use truestrike you can change the weapon damage to psychic, but the truestrike damage must be either what the weapon does or Radiant.