Journal changed the ordering of the authors. What to do now?
You seem to be in a Math department, and are having a problem that is going to effect everyone in that department.  I suggest you speak to the chair of the department.
There are journals in mathematics, good ones, that insist on an alphabetical order of authors.  This is also the standard in most of pure mathematics.  If you have your facts correct, your university has policy that will make it very hard for anyone in pure mathematics to publish.  Applied math is a bit different but this still will be a problem.
As an example, the journal Results in Mathematics explicitly requires alphabetical author order.