Sorry to read this.
Contact Union - essential. Ask their advice. When I was being shafted they came round and talked for ages. Turned out Head had a record for causing upset and Unions had her marked.
Take a friend / union rep into meeting ( they might not be allowed to say anything but they are A: a witness B: show that you are serious.
Take notes in meeting, control speed of meeting, clarify any points made, reiterate what has been said.
Write your main points down before you go in. Be clear in what you want.
Don't loose your rag or cry. Keep calm and be the personification of reasonableness. Remember that getting your own was in the classroom is a way of life. Your SMT are just big kids and need dealing with.
Don't agree to anything there and then unless you are 200% happy with it. Ask, if you are being pushed, to let them know after, say, the weekend.
Ask to see policies - they may be making them up on the spot.
You may also be able to challenge what you 'agreed' to. Was it verbal or in writing?
Sadly your previous LA conditions may not apply. I am in a former LA comprehensive that decided to go down the academy route. We aren't in a group nor were forced.
My terms and conditions were tuped(?) across and applied for a year. Now they can adjust/alter/rewrite them. So far we have had few changes but some look to be in the pipeline - loss of thresholds etc.
Not that it will affect me! I'm retiring in 14 days. CP