Can a Fixed-Price Request be switched from a Bid/Budget to a Milestone after posting a request? If so how is it done? Also, can it be changed after hiring? If so how?
No, You can't change fixed price jobs to milestons and vice versa. You can change the price though, by clicking on the button "Change Terms". You can find it under Your "Proposals", then choose the job You would like the terms to be changed in. After You are hired, I believe that the conditions can't be changed. After the completion of a job, You do have an option to give Your client a refund.