Could this be done?

[proneal]

Select Case Form2.CheckBox18.Checked
            Case True
 
                lines(foundIndex) = Regex.Replace(lines(foundIndex), "(?<=" & "sIntroSequence" & "=)\d+", "BGS_LOGO.BIK") 'WRITE "BGS_LOGO.BIK"
                IO.File.WriteAllLines("C:\test.txt"), lines)
            Case False
 
                lines(foundIndex) = Regex.Replace(lines(foundIndex), "(?<=" & "sIntroSequence" & "=)\d+", "") 'REMOVE TEXT (IF ANY PRESENT)
                IO.File.WriteAllLines("C:\test.txt"), lines)
        End Select

You are checking the wrong property in your Select Case statement

One of the earlier posters resolved your problem already.
Sorry you weren't paying attention