The easiest way would be to hide the lastname field and change the wording of "First name" to "Full name"
Note that you'll have to do this in MANY places (registered checkout, guest checkout, account create, new address, etc)
1. EDIT EACH OF THE FOLLOWING:
And change "First Name" to "Full Name"
2. EDIT EACH OF THE FOLLOWING:
And remove any lines with reference to "error_lastname"
overall it just isn't worth it.