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?

Yes - That's exactly what I'm trying to do. I have other 'Replace()' functions running in the same Workbook, and they run fine when running a single term - like this...

    Term = Range("C1").Value
    CertificateDate = Replace(Term, "Term", "Certficate Date")

It works fine...

Did you try what you suggested? It's not working for me?

Can you show me the full block? Or was it only that line you replaced?

When I add the line you suggested, this is what my block looks like...

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

Is there something I'm missing?

Nevermind. I've figured it out. I removed everything else and just used what you suggested. It was simply to cluttered with code. Thanks for your help!

Is there something I'm missing?

Yes. Your last lines are referencing the wrong variable. It should instead be:

Range("A1").Offset(, 0).Value = LAST_NAME 'Note: the offset is pointless in this situation
Range("B1").Offset(, 0).Value = FIRST_NAME

With that being said, you should only need these 2 lines (tested and worked).

Sub B_ReplaceHeaders()

Range("A1").Replace What:="Last Name", Replacement:="LAST_NAME"
Range("B1").Replace What:="First Name", Replacement:="FIRST_NAME"

End Sub

Thanks for the reply. I've added to your rep. Much appreciated!