An 'AGREEMENT' constitutes a contract. A contract (usually) does not require to be in writing - and certainly not in an employment contract.
If terms are unwritten it can pose a problem - due to lack of proof. So what the Courts do is look for 'implied' terms and conditions. They look at past history to identify implied conditions. That past history shows that you were always paid X to do Y. Now your employer is offering to pay you W to still to Y (as it appears to me). You don't seem to agree with this change, so it's a unilateral change - and that's a breach of contract. Furthermore since it directly relates to 'pay', it's a material breach. So what do you do?
Well, doing nothing implies acceptance. So if you really don't want to accept this change you have to fight against it - and unfortunately that takes some courage - especially if your on your own.
If you are one of a number affected (as I suspect), and you all feel the same way, then stick together. If you do, you'll find your almost there (in my experience).
If you are in Scotland you can 'implement' your employer to meet their obligations. If in England/Wales you cannot. But you can claim damages. Unfortunately this means you basically have to resign - your damages will be whatever it costs you in loss of pay. You are obligated to minimise your loss by finding another job. Employment law/ Contract law is unfortunately written with the employer in mind rather than the employee. Eighteen years of Margaret Thatcher saw to that!
Cue the right!!