in what way? they could bring the 10,000 down even further?! or are you referring to the starting and finishing times?
Yes, they could change the salary, or they could decide that the September course satisfies all the requirements for trainee ATCOs for the next three years, and further courses are cancelled.
I'm not saying any of those will happen, or are likely to happen, but they technically could. It's a risk. Ask those who deferred entry to NATS and got stung by the reduction to 10k...