I'm doing my best to figure out how to create a weapon that has two forms and thusly two damage die. It's base form is a short spear (Yklwa) and it's empowered version is a spear that deals 1d10 damage. So my question really is how to I create weapons that have a base weapon but upgraded damage die? I can't seem to find a way to change the spear's 1d6 (1d8) damage die on the character sheet through editing the magic weapon.

Ideally this is one magic weapon that with an action can change between these two forms, but simply being able to change the spear's damage die would be helpful in and of itself.

I would also like to know how to do this. I am looking to create a +1 spear of elven make that tranforms into a +1 Longbow. The only work around that I have found so far is to create 2 seperate items for the player to use in their dndbeyond character sheet. 

Best thing to do is find a weapon that has the same damage rolls as what you want to make and add all your flavor text in there. At the end of your custom description saying "Ignore everything after this point" as the last line because there is no way to eliminate the default text that comes with a weapon which is displayed below your custom text.

And you'll have to get creative with proficiencies if this weapon is given to a character that doesn't have all simple or all martial weapons as a proficiency.

Quote from PFD6 >>

The only work around that I have found so far is to create 2 seperate items for the player to use in their dndbeyond character sheet. 

 I think this is going to be the best approach really, both in terms of work you need to do and ease of use on the player side. Just tie them together through the name like Treestabber [bow] and Treestabber [spear].