Call jscript from workflow?

Hi I am trying to update 1000 contacts with the following jscript.

function LastName_OnChange() {
Xrm.Page.getAttribute("new_3lettername").setValue(Xrm.Page.getAttribute("lastname").getValue().substring(0, 3));
}

If I set this to onload event then the field isn't actually populated until i go in and save the record individually. If i try and make this an onchange event from a different field to trigger it, it still doesn't work unless i go into each individually and change then save. How can I do this in bulk without having to go through and individually save my 1000 records and all in the future.