When both parties agree to definitize a contract change before the work is begun, the change is made contractually through a

Supplemental agreement

Explanation:

A supplemental agreement is an agreement which is modified (but not replaced and/or rearranged) with the mutual understanding and consent of both the parties involved. The binding spirit of the contract is not affected because of this modification, nor the mutual considerations to be transferred. The reason behind a supplemental agreement could be anything, it might be because for the inclusion of some important consequences that were previously left unmentioned, which could have increased the contractual inadequacy risk for both the parties or any reason detrimental to legal and/or financial capacity of the parties involved.

one of the appeals of a sole proprietorship is that business expenses are kept separate from your personal income tax return