Amending a text file

Just read the file as one giant string. Then run replace.

It isn't clear what your exact requirement is... but something like below for single file read, replace string(s) and write back.

Sub Demo()
Dim iFile As String: iFile = "C:\Test\full-20190501.txt"
Dim intFF As Integer: intFF = FreeFile()
Dim textData As String

Open iFile For Input As #intFF
textData = Input$(LOF(intFF), #intFF)
Close #intFF

textData = Replace(textData, "Original String", "New String", , , vbBinaryCompare)
Open iFile For Output As #intFF
Print #intFF, textData
Close #intFF
End Sub

EDIT: FYI - Tested on 34Mb log file with 250k+ rows. Took about 3~4 sec. Combine it with DIR() function to iterate over multiple files if needed.