This is like any change to Employment Terms; it is a change to your contract of employment and must be agreed. You would be best advised on this by someone familiar with employment law (HR, Trade union, employment law specialists).
In my experience, where the circumstances dictate change, there isn't much that can't be changed, however what does appear unusual in your case is that the changes were unevenly distributed. ("Some people had pay cuts. Others were unaffected, and their employer contribution of 5% (or more) was left untouched.") This may be worthy of further investigation.