The change to an 0700 is legal.
If you want a life then join a decent airline, where changes like that are catered for in a written agreement.
In my airline I would be phoned at a decent time of the day and asked if I can accept a change. If can or can't, I say so. They may try someone else or possibly buy back a day off from a willing volunteer.
If you don't want to change airlines, join Balpa and persuade others to join as well. You will then be in a position to get niggles like this sorted out.