How do I Flash Samsung Galaxy S5 from T-Mobile to AT&T with Odin and Stock Roms?

[AC Question]

I have unlocked my phone from T-mobile. It is a stock Galaxy S5. Not rooted. I want to be able to give it to my mother-in-law to use with AT&T. I have down loaded Odin 3.10.7. I have the stockrom for ATT. The drivers are up to date on it. Windows 7 64bit ultimate. I have tried to updated it but it just keeps failing. Says this in Odin:
<ID:0/003> Added!!
<OSM> Enter CS for MD5..
<OSM> Binary Check MD5.. Do not unplug the cable..
<OSM> Please wait..
<OSM> Checking MD5 finished Sucessfully..
<OSM> Leave CS..
<ID:0/003> Odin engine v(ID:3.1100)..
<ID:0/003> File analysis..
<ID:0/003> SetupConnection..
<ID:0/003> Initialzation..
<ID:0/003> Get PIT for mapping..
<ID:0/003> Firmware update start..
<ID:0/003> SingleDownload.
<ID:0/003> aboot.mbn
<ID:0/003> NAND Write Start!!
<ID:0/003> FAIL! (Auth)
<ID:0/003>
<ID:0/003> Complete(Write) operation failed.
<OSM> All threads completed. (succeed 0 / failed 1)

I have made no changes to Odin. I am using the AP field. This seem straight forward to me but somehow it is not working. What am i doing wrong? What do i need to do here to make it switch? Thank you!

 

[dpham00]

Mod Note, I will move this to the s5 forum for more visibility

 

[ironass]

I have unlocked my phone from T-mobile. It is a stock Galaxy S5. Not rooted. I want to be able to give it to my mother-in-law to use with AT&T. I have down loaded Odin 3.10.7. I have the stockrom for ATT. The drivers are up to date on it. Windows 7 64bit ultimate. I have tried to updated it but it just keeps failing. Says this in Odin:

That's interesting, as the latest, Android 4.4.4, 5.0 & 5.1.1, AT&T firmwares for the G900A model are not generally available in a .tar, Odin flashable, format. Have you got a link to the download for this firmware?