Many thanks for your reply, for anyones info:
//The new filter to use
IExpression updateExpression = new Expression(CustomerOptionFields.Cop_checkoptions, ExOp.BitwiseAnd, ~filter);
CustomerOptionEntity options = new CustomerOptionEntity();
options.Fields[(int)CustomerOptionFieldIndex.Cop_checkoptions].ExpressionToApply = updateExpression;
CustomerOptionCollection multiOptions = new CustomerOptionCollection();
// no filter is specified, everybody gets 10% extra, but you could of course
// specify a filter to limit the scope of the update.
multiOptions.UpdateMulti(options, null);