My large Employer is changing paying staff from every four weeks to once a month, to change this they have ded?

My employer is changing paying staff from every four weeks to once a month. During the first month change they have deducted two days/shifts and have told us that those two days owing will be added on to our week in hand money ( 7 Days ) to make 9 days owing when we leave. They informed us this via letter in post a couple of months ago. Are they legally allowed to withhold wages and simply add on to monies owed when an employee leaves ? We all have contracts which seem to suggest they can alter or amend terms accordingly.