How do i move data from one field to another automatically?

I am making my company's forms to be more relationship based rather than everything being a simple text field.  I built a relationship between two tables called Support Tickets and Contacts.  On support Tickets, there was text field called "Apt#".  I created a new field called "APT#" which is now a text lookup field.  So when the field "full name" (a text lookup field and relationship to full name on Contacts table) on the table "Support Tickets" selects a customer's name, it will automatically fill in the field APT# in the table "SUpport Ticket".   How do i have the information from the old text field Apt# move the information into the new field APT# on the table "Support ticket"?  Could i have modified the old field "Apt#" to be a text lookup?  If so, wouldn't all the original data in that field be lost?  Thank you.