Yes an IEP can be changed without the parents consent in most places.
The parent though has to be notified in advance of the IEP being changed but in most places the IEP can be changed without consent from the parents.
The school must allow the parents to discuss any changes of the IEP before the school can change it but even if the parent doesn't want the IEP changed the school can change it.
Although in my opinion the school should require the parents consent for changing the IEP but in most schools and school districts it's not required although it should be.
Still the parents have to be notified beforehand or the IEP cannot be changed and if they change the IEP without notifying the parents then the parents can challenge it.