Update Lead Source Original with Lead Source Most Recent if empty

Is there an easy way to update the Lead Source Original with the data in Lead Source Most Recent?

I'm thinking in a form fill Lead Source Most recent should always be filled out when the form gets submitted so its updated. But there should be a program to check that if there is a lead source most recent but not a lead source (so in this example this is the first time we see this lead) then the lead source original should be updated with the lead source most recent.

Whats the best way to do this so that its scalable and never something I have to worry about as we add more content.

Thanks!