Can a workflow populate values in a lookup field for all the items in the list?

I have a list called Contacts, with 4000+ entries. It currently has a dropdown choice column with four specified values for different offices (MAO, NHO, MEO and CTO). I want to convert this dropdown choice column to a lookup column that links to a newly created list with the four offices in it. The reason for this is because I want to use Mark Rackley’s cascade dropdowns to filter Contacts by office.

From what I understand you can’t directly convert a Choice field to a lookup field. So I thought I’d just create a new lookup field that links to the new list with the offices. The problem is there are already 4000+ items in this list that would need to have the applicable office chosen in this new lookup field.

Is there a way to create a workflow that will look at the value in the choice field and then select the matching value in the new lookup field for all the contacts? For example, it sees MAO in the choice field and then selects MAO under the new lookup field.