21.02 changes

gistfile1.txt

Sysupgrade can be used to upgrade a device from 19.07 to 21.02, and configuration will be preserved in most cases.

:!: Sysupgrade from 18.06 to 21.02 is not supported.

:!: There is no migration path for targets that switched to DSA.
In that case, sysupgrade will refuse to proceed with an error message: 

Image version mismatch. image 1.1 device 1.0 Please wipe config during upgrade (force required) or reinstall. Config cannot be migrated from swconfig to DSA Image check failed

The user has to force sysupgrade and reset the device configuration.


(notes:
unsure if we should mark with :!: 

text from link to example from user of WRT1900ACS : https://i.imgur.com/bjq4wzf.png
- this user came to irc to get help regarding that message

Is there a list of DSA / affected targets / how much devices are affected -> reorder stuff to top / add some info there (first paragraph)?

maybe improve text description of what works / does not work ? - I dont know if these are correct ! 
- "force sysupgrade"
- disable keep settings 

minor nitpick : the luci error message misses 2x the end of sentence dot (DSA. Image check failed.)
but cannot find these strings with github search function
)