Using Replace() to search for a two-word phrase

When I run the following against a single word, it runs fine, but I need to run it against a two-word phrase. I'm getting a bug when I try.

Sub B_ReplaceHeaders()
    
    'Replace Last Name with LAST_NAME
    Dim LastName As String
    Dim LAST_NAME As String
    
    'Replace First Name with FIRST_NAME
    Dim FirstName As String
    Dim FIRST_NAME As String
    
    
    LastName = Range("A1").Value
    LAST_NAME = Replace("Last Name", "Last Name", "LAST_NAME")
    
    FirstName = Range("B1").Value
    FIRST_NAME = Replace("First Name", "First Name", "FIRST_NAME")
    
    Range("A1").Offset(, 0).Value = LastName
    Range("B1").Offset(, 0).Value = FirstName

End Sub

Any ideas on how I might account for the space in the search string?