If your hardware supports 21.X then you can upgrade from 18.104.22.168 to 21 without any issues.
Downtime will depend on your process of upgrading and (again) the hardware platform. What is the hardware you are trying to upgrade?
There is no issue with pre-loading the image and then waiting for the maintenance window to reboot the switches. Using this knowledge article: https://gtacknowledge.extremenetworks.com/articles/How_To/How-to-Obtain-and-Upgrade-EXOS just d...
The file you downloaded was the MIB file for that version of EXOS. It does not contain the image of the firmware.
The file you want to download is summitX-22.214.171.124-patch1-4.xos . It will have the ".xos" extension originally.
The difference between these two cards is a functionality called
Synchronous Ethernet (SyncE). The XGM3S-2sf (16126) supports SyncE while the XGM3-2sf (16117) does not.
SyncE is standard for transmitting a synchronization signal or clock signals o...