Can employers change a commission scheme the month AFTER you have hit targets?

I work for a mobile company in a call centre on a sales team. The last few months we have been targeted to hit a conversion rate for the types of calls we were getting. Now, after hitting required targets, 8 days into the month after my employer has decided (to do this to over 300 members of staff) to changed the target. So lots of us will now get over �200 less money in our account this month than what was expected. Are they allowed to change this at a whim???