can the gx450 verizon (MC7355) be used with at&t by upgrading its firmware??

Yes it can. I just did one today on the GX450 Verizon model.
Download the package for AT&T, link:
source.sierrawireless.com/resou … s-for-att/

It is a zip file and contains 2 files. ALEOS & MC7354 Radio Module Firmware
Unzip the file once downloaded.

First check if you have 1 network operator listed:
From the ACE Manager, Admin tab-> Radio Module Firmware - this should show VZW (Verizon) as the active.
Then click on Software/Firmware (at the top). Select Radio Module Firmware(from the pop up box) and hit the browse button and browse to the file MC7354 Radio Module Firmware you unzipped). Then hit update.
The image should load, and the device will reboot. It takes about 30 mins.
After rebooting, you can check in the Admin tab-> Radio Module Firmware - and it should show the AT&T.

If you see 2 network operators listed, example Verizon and Generic or Verizon and Bell, you’ll need to remove one and follow the steps above.

The full instructions with pics starts on page 24 of the instruction guide.

Hope that helps.