The school cannot withdraw your statement without clear evidence, and if they try to withdraw at AR, you can appeal. They really will not have anything to back their position up, and tbh they are foolish if they try to change the number of hours.
We're in a sort of similar position, in that we don't have ABA exactly funded through our statement, but have it written in (after appeal, obviously) that the TA will be trained by our supervisor, and will continue to work to jointly agreed targets.
We also had our own tutor going in one day a week, but she has just left on maternity. Needless to say, it has been a battle for 2 years to ensure that school do not just take over and ignore any ABA input, and I think without our tutor (and the current TA leaving) it will get worse, but as it stands, his current statement continues. As you have said, this is what you need to concentrate on. It is really hard though, esp if the LSA is partly employed by school, partly by you, and it sounds like (like us) school don't really want ABA, but have to because of the statment. Like you, it's not a question of funding/number of hours for our school, but an ideological (and obviously ignorant/prejudiced) opposition to anything that they don't control.
Must've been a slight shock for DU (we used him too) to say that. But then our consultant said that ds could 'wind down' ABA two years ago! And it didn't mean it goes from ABA -> nothing.
You can ensure that the approach is consistent - ie the school really cannot justify at all going from full support to nothing, and a switch in the type of support can be argued to be extremely detrimental for dd. Has DU written his report yet? Can you ask him to include in it that even though a full ABA programme not necessary, the positive-reinforcement approach is still essential? This is kind of what he said for us - we were never going for a full funded programme, so he said that the approach used so far was so successful it would be the most appropriate form of support to continue with. And presumably you'll have data to back up the areas of support dd has had, progress made on them, and areas which still need help with.