The LA should email you an electronic copy of the Final Statement. This will, in effect, be version 1 of the WD (but, in reality, will be an exact copy of the Final Statement before appeal)
You then start work on your version and this will become Version 2.
On your version 2, put in your changes with the parent's markup formatting. Footnote every addition you make so that it refers back to one of your (submitted) reports (the footnotes will get removed on the post-hearing Statement but pre-hearing you don't want the LA to accuse you of making stuff up).
My LA didnt engage at all in the process - except by sending the electronic copy. Instead they turned up on the hearing day with the majority of my version 2 agreed. Because they did this, we has to delay the start of the hearing by nearly an hour to give me and my team time to read and digest their new version. The LA were not told off at all for not engaging with us. The judge just looked resigned that this was something she was used to.
There is a timescale for at least the first changed version of the WD to go back to Tribunal Services (so it can get included in the evidence bundle) - I think it's about 2 weeks before the hearing. But if the LA does engage with you, you can keep going upto and including the hearing with each different version.
Word of warning. Watch out that the WD doesn't become corrupt. It is a very flaky Word document which many inexperienced people will have worked on. In my DS case, we went to Tribunal with version 2. By the time the Judge had ordered everything, we got to version 9. These extra 6 versions were only to correct numerous LA typos. Somewhere along the line, the wrong formatting got "caught up" in the document making it look as if we had conceded stuff. We hadn't - the document had become corrupt. The cynic in me would say that the LA purposely caused this - but they hadn't - an inexperienced LA typist had worked on it and used the wrong formatting. It took some untangling on my part and didn't help post-tribunal nerves!
When I complained about my LA to LGO, because we were at version 9 when it became the final judge-ordered Statement, the LA had the cheek to tell me and the LGO investigator that they had fully engaged with us prior to the hearing because there was 9 version. I wasn't impressed!