Thank you for your response.
My question is about whether there is an operating agreement, or simply a LLC certificate of formation. However, based on your answer I assume there is only a certificate.
As that is the case, a simple document between the two of you which states the new income split, is signed and dated, will suffice. It also needs to state that the liability for payment of taxes will also change to the 80/20 split, instead of 50/50 (if that is the arrangement). This document does not have to be filed with the state and must simply remain in the company records.
In regard to most of the work taking place outside of California, as long as the income is paid to the Californian LLC, then this is not an issue. However, because the work is outside of CA, you might look into setting up another corporation
outside of the state to receive the payment, allowing you to lower your franchise
tax burden. This will depend on if the work is going to be worth enough to adequately cover the transaction costs involved in setting up the out of state company.
Please let me know if you need further information or would like to discuss this matter further.