Change of policy of company sick pay scheme?

My company have for the last 30 years operated a company sick pay scheme whereas you would receive up to 20 days sick pay. Although at the end of the entire contract it is written '****** reserve the right to make make changes', they have not been specific to what changes, (does that give them right to change things willy nilly?) and after 30 years of sick pay would it not be implied that it has become custom and practise and therefore would form part of the contract? However, they have now introduced without agreement from their employees, that after the 3rd sickness, sick pay would not be paid (even though they are operating it as 2 times off, then lose pay). Are they able to do this legally or are they in breach of contract? Thank you