Thank you for your response, but you may not have not understood my question because your response does not appear to address it. For example, when you wrote "Since your first set up was Inventory Asset, it's the reason why your build assemblies showing the inventory asset account.". Yes, you are correct, and I already know that. What I'm asking is how do I change that?

To be clear:

How do I change the account that was used for an existing "Build Assembly" transaction that was already posted? I am trying to change the account for HISTORICAL / EXISTING build assembly transactions that were already posted. I know that the transactions will change going forward - I want to change past transactions.

Changing the income account or asset assigned for the either part item or assembly item did not work. 

Please know that when you change an inventory asset account on an item, the transactions related to the Build Assembly will reflect the account after the changes.