I've upgraded lots of device models to ROS7 using the file upload or the Webfig Package->Upgrade methods.
Our CRS326 (CRS326-24G-2S+) and CRS317 (CRS317-1G-16S+) always say after reboot they haven't the needed memory for the upgrade and keep running ROS 6.49
I've only been able to upgrade that two device models using Netinstall and that can't be easy done remote and requires a lot of configuration restore work.

Is there any other option to upgrade this devices from 6.49.x to ROS 7?
Some temporary spacial image to allow the major version cross?

thanks in advance!

No.

If work, why update uselessly?

Just out of curiosity, what do you want to achieve by upgrading major version?

The reasons are:
* Not being stuck in a version out of active development.
* Benefiting from the features added in ROS7
* Harmonizing the monitoring across all the devices. The API structures aren't the same in ROS6 and 7