Imo, changing retirement on those already under contract is a violation of that contract.
If soldiers have no reason to maintain good faith and confidence in their government, then the government should be terrified of having to try to maintain good faith and obedience of the soldiers.
Wouldn’t be the first time. I was told at my first reenlistiment I’d have free health care for life if I stayed in for 20 years.
By the early 90s, that promise was “retired”.