Its unlikely the statement has changed as that has to go through a formal process and you would be told. Whats probably happened is the Council has changed the threshold for top up funding and is expecting the school to pay a greater % of statements from its delegated SEN budget (element 2 funding).
Funding changes in April 2013 mean schools have to pay the first £6000 of each statement now. Some Councils are only starting to bring in the change. Or for ABA some councils might have paid the whole cost as an 'education otherwise than at school' package, but now be asking the school to pay part of it. Councils are wanting to get schools to pay £6k per statement wherever they can.
Some councils also have reduced the amount they pay schools per hour of top up funding (element 3 above £6k) so for e.g. my LA used to pay schools £500 per hour on a statement so if a child had 10 hours 1:1 top up funding the school would have got £5000 pa. Now the council has changed the rate to £460 per hour so schools would only get £4600 and be expected to pay the rest from element 1 or 2 funding.
Element 1 & 2 is based on a formula e.g. deprivation, free school meals etc and not on how many children with sen the school actually has. Schools have also got used to thinking of this as 'their' money to spend how they like and are now reluctant to ring fence element 2 for sen, and particularly reluctant to ring fence it for a particular child with sen.
IPSEA explains funding here
In DS old school when the changes came in the school had to get rid of several class TAs because the funding had come out of the general sen pot and now had to be spent on SEN support for individual children (because the Council stopped paying for the first £6k of their support).
I would consider making a FOI request of the council for your file and also from the school asking to see all the docs about statement and funding.
The chances are the school can pay your ABA tutor but doesn't want to because that would mean they would have to use money currently spent elsewhere. But if your statement says ABA then the LA must find a school prepared to accept ABA and must pay what ABA costs.
I doubt the statement has been reviewed, I suspect this is an internal argument about how the statement should be funded. Sadly many schools are not wanting children who need 1:1 support now they have to pay the first £6k.