Moving away from a fixed scope for a fixed price is a legal obligation, so the process I recommend in is simply that - keeping the contract as broad as possible

Moving away from a fixed scope for a fixed price is a legal obligation, so the process I recommend in is simply that - keeping the contract as broad as possible to write what the legal obligations of both parties are. Instead of stating the obligations are 'X scope for $Y", I advocate "$Y", which is actually less detailed by intent.