for your solutions your need to find the list item from check box list which value is same as your previous value and set its selected property to true or selected, then it will checked/unchecked that check box.
some thing like this...
chklist.items.findbytext('yourpreviousvalue').checked = true;
please refer this link it will demonstrate you how to check/unchecked check box list item based on your condition.
Check/Unchecked Check box list