Can anyone please tell me if there is a way to change the base damage or die, on a weapon you create?

ex; A player in my campaign bought a Knife that does 1d8 slashing instead of the typical 1d4 with a dagger. 

For the sake of my sanity please tell me how to change this so the 1d4 attack option isn’t visible anymore. I’ve been trying every option i can think of as far as modifiers, I can’t take it anymore. 

On a related note, if this just can’t be done & I'm forced to create this “Chefs Knife” using a Longsword as its weapon base.. that would also be very welcomed information. Thank you so much

If this is to be an item you may use in future: no. 

If this is a one-off item, the player can make a custom action to achieve this.

The easiest option is just adding a longsword and renaming it. Does everything you want.

It is not possible to change the base damage for a weapon. Just use the Longsword, but add the following Modifier:

Modifier: Ignore Weapon Property->Subtype: Versatile

Yea this is where i ended, Longsword base with the ignored is as close as i could get. Thanks all

Quote from Cyb3rM1nd >>

 The easiest option is just adding a longsword and renaming it. Does everything you want.

The longsword has the versatile property and slashing damage, whereas a dagger has the finesse, light, and thrown properties and piercing damage. Using the modifiers "Weapon Property", "Ignore Weapon Property", and "Replace Damage Type" could be used to make the necessary changes to the longsword to match up with the custom dagger, though they won't be able to put in the thrown ranges. 

They could use a yklwa. It is a d8 weapon that has the thrown property, though it's range is different from a dagger's. The yklwa is the only d8 weapon that already has the thrown property so either option (this or the longsword) works out the same in the end.

DadStad, give this a read through.