Sourcing Field, Then update the criteria.. Easiest way to update records?

We have custom fields (custfield_1) with custom records (custrec_1) that are being sourced from custom field/record (custfield_2/custrec_2).  Now we updated the criteria (custfield_2) and I am trying to think of the easiest way to update the existing (custfield_1) records so they are being sourced correctly.

I am pretty sure I could create a custom mass update to update the existing records, that would be similar to the reassign territories feature.  I know I could do an export, then re-import back in and update.  Definitely don't want to do this.

I could envision creating a script that would open up all the records then capture the 'custfield_1' then add the same field to the record then save it.  Essentially doing like I would manually open the record, click the again then it would be properly sourced, then save the record.