Amendments are fine, however they should be tracked as project change requests so they will not disrupt the current plan. Of course I've had clients insist they know better and you may have to take the hit, as long as the client knows they are changing the outcomes and it may cost them more. Not sure why this would be free work for you, so perhaps you can leave a comment and I will email you back to continue the discussion. If you are being compensated as the PM, tracking and managing project change requests should be work you are compensated for. If you are an independent contractor, you may need some additional items added to your contracts or SOWs in the future.