I'm in a very similar position to you, zazzie - the timings here are a real grey area, and our LA is gleefully exploiting it.
As things stand, Chapter 9 of the 2001 SEN Code of Practice has the guidance that applies now. The guidance isn't very clear though when it comes to your issue.
You should have had a copy of the head teacher's report from your Annual Review meeting by now (10 school days after the meeting). If it's an accurate account of what they said at the meeting, it should be recommending further amendment to the statement - hopefully recommending specific changes to Part 4.
This is where things get blurry. The LA are under no specific time pressure to make a decision on amending the statement - but when they do make a decision, they have to let you know within a week.
There's a sentence in Chapter 9 of the 2001 SEN CoP (9:35) that says "Where the decision is to amend the statement, the LEA should start the process of amendment without delay." Unhelpfully, this is a "should" not a "must", and there's no time limit specified, or what "without delay" actually means in a practical sense.
We're waiting for our amended statement (changes to Parts 2,3,4). We have been waiting for several months. The LA is interpreting "without delay" to mean "for as long as we can get away with before even the average Zen Buddhist starts to think we're taking our time"
This is one area where the new draft Code of Practice looks like it's going to be a bit tighter - 4 weeks for a decision to amend or not, and 8 weeks from there to issue the amended EHCP.
The one thing that might make a difference is if your DS is about to go into a major phase transfer, like primary to secondary, or Year 9. I think different timescales apply here, but I'm not sure.
Hope this helps!