Actually, my experience is that with individual packages installed (in v6) it is more likely to fail to upgrade to v7 than with the bundle package.
To resolve the issue, use netinstall to install v7.
I can’t netinstall hundreds of devices in the field. Nor can I deploy them or upgrade them remotely if upgrades even between minor versions are prone to failing.
It is not a good idea to mass-upgrade such tiny devices from v6 to v7. There is quite a risk of problems, and performance will be worse.
Remain on v6.
Unfortunately the functionality that I require on the wAP R ac to make it compatible with the RB922s is not working on ROS 6. And I can’t downgrade the wAP R ac to the version where it is working, so I have to run ROS 7 or there is no compatibility. MikroTik have also mentioned they have no plans to backport the fix to ROS 6.
Last week I update my hAP mini (smips) with 32MiB of Memory and 16MiB of Disk from v7.10.2 to v7.11.2. This was without any Problems. I only have to clear all Files in the “File-List” ( /file print has to been empty) Only the directory “skins” is listed. I didnt reset the device. The complete configuration stays on the device.