So, upon completion of the full Key Field, (First Name & LastName) you want the rest of the form to be completed from the existing record that matches First Name & LastName? Why not set a self-relationship using that key field match, then set the form fields to be lookups? You can still over-type them if there is no match, or you want to edit them.
Yes, after Last Name, if the record exists fill in all the fields including First and Last Name as well as the others. Do I need to convert all fields of the record in lookups. How do I convert the fields in lookups (Inspector / Properties: Edit Box, Auto-complete using existing values ?)
In each of the relevant fields' definition, set it to auto-enter by lookup, using that relationship, and its matching field's contents.
But I have a worry: "if the record exists fill in all the fields including First and Last Name"
You've just filled in the First Name and Last Name for that record...