You will absolutely need to reset the settings to defaults (and do not attempt to restore a backup from BB to 21.02 -- it will not work).
The upgrade involves also involves a significant change to the target architecture (from ar71xx to ath79 -- it is a change to the way everything under the hood works while still targeting the same hardware).
I'm pretty sure that you cannot do this in a single upgrade, but I'm not 100% certain if you'll be able to do this in two steps or if it will require more. I'd recommend upgrading to 19.07/ar71xx first, then running the upgrade to 21.02 after that.
For the most likely success, you should probably reset your router to defaults before running the upgrades. Then, use the sysupgrade images to go from BB > 19.07 > 21.02. If you use the LuCI web interface, you will be presented with the option to keep settings -- do not keep settings for either one of these steps. If you use the CLI, be sure to include the -n switch (which specifies to not keep settings).
When you upgrade from 19.07 > 21.02, you will be changing the target architecture and you will get a scary looking warning. It is okay to proceed (again, do not keep settings).
It is best practice to make sure that the checksum matches what is published on the firmware selector page -- this way you can be sure that the file has not become corrupt along the way (this is rare, but can happen).
Because sysupgrade mechanism (image metadata) changed somewhat in 2016, I would first sysupgrade to 17.01 , in order to get the new logic for the later sysupgrades, and then probably to 19.07, still with ar71xx architecture.
All sysupgrades without keeping settings...
(So jumping over 15.05 and 18.06)
Then sysupgrading to ath79 architecture 21.02. that needs "force" option as ar71xx/ath79 causes image verification to fail.
The other alternative would be to read the advice from the device wiki page, and take the bootloader recovery flash route directly to the 21.02.1 factory image (and then for some strange reason also reflash with a sysupgrade image).
fyi, I think 21.02.1 may be broken for MyNet N750.
I have a MyNet N750 running OpenWrt 19.07.7 ar71xx. It had been upgraded over the years from 18.06, 17.01, keeping settings each time.
I just flashed 21.02 squashfs-sysupgrade using LuCI without keeping any settings. I was able to log into LuCI, but when I selected 'Reboot' in LuCI, the router restarts but LuCI fails to load.
I can SSH into the router. I tried forcibly unloading LuCI and reinstalling LuCI but it did not resolve the problem.
Next, I scp sysupgrade.bin and ran sysupgrade -n -F. Router rebooted and I was able to log into LuCI again. But as soon as I reboot, LuCI fails to appear when router starts up.
Next, scp sysupgrade.bin and ran sysupgrade -n -F. Router rebooted and I was able to log into LuCI again. I then installed sysupgrade.bin file again using LuCI. But as soon as I reboot, LuCI fails to appear when router starts up.
Next, I used Emergency Room recovery method and installed the 21.02.1 squashfs**-factory**.bin. Router rebooted and I was able to log into LuCI again. I then installed sysupgrade.bin file using LuCI. But as soon as I reboot, LuCI fails to appear when router starts up.....
Looks LuCI is broken in 21.02.1 for MyNet N750 to me?
Update: My tests seem to indicate 21.02.0-rc2 is last release where LuCI can survive a reboot.
Thanks to all of you for some very concise and detailed information. I'll have to digest it all and determine my best plan of attack
One additional question that I should have included in the original post...
What are the overall benefits of doing the 14.07 > 21.02 upgrade? My initial reason for investigating this was for stability issues I've been having. And hoped this upgrade would help alleviate some of the WIFI outages I occasionally have. Not terrible, but I do have to manually reboot about once a week. This is in addition to the automated reboot that I have running every night.
This part is huge...
14.07 is extremely old, unsupported, and has many known and actively exploited security vulnerabilities.
Upgrading to 19.07 or 21.02 gets you back into supported territory (although technically 19.07 will go EOL in the relatively near future), with up to date security patches, new features, and up-to-date offerings with the package manager (should you choose to use them)
If your issue is related to the stability of the firmware itself, 19.07 and 21.02 seem to be pretty solid for most devices. However, I don't know how many people are using your specific device, and per the comment from @bill888, there could potentially be some issues with that unit (but @mk24 points out that it could be browser based -- we'd need additional datapoints or experiments to know for sure if @bill888 's experience actually indicates any real issues)
If your situation is caused by failing hardware, though, you will be best served by replacing the unit.
Sounds like maybe going from 14.07 > 17.01 > 19.07 to start would be my best bet. And probably the lowest risk. I can see if that's enough for now at least. Then I can decide later if I need/want to jump to 21.02.
If I sysupgrade from 21.02.0-rc2 to a later release such as 21.02.1 and 'keep settings', I have observed LuCI survives subsequent reboots (until I reset the device to default OpenWrt settings)
I also observed when I back up the settings while LuCI survives reboots, and then restore the settings, I get 'LuCI Configuration interface' followed by "Bad gateway". WinSCP then refuses to connect until I reset the router to restore openwrt defaults.
I couldn't find any differences between the files in /etc/config. Only the ULA prefix in /etc/config/network differed.
21.02 seems to be snafu on MyNet N750....
I've put 19.07.8 ath79 image on the N750 and it doesn't exhibit the LuCI or backup/restore issues so far. I know 19.07.7 ar71xx image works reliably.
Might be about the host ssh certs and the SSL cert for LuCI (for uhttpd daemon). Browsers (and SSL enabled tools in general) are sometimes really picky about connecting to the same host if the certs have changed during a browser session.
I could not login via LuCI (correct password brought you back to the login screen)
after I SSHed to router and reboot the would not boot back up
I had to use the Emergency Room firmware recovery to downgrade to 19.07.9.
(Also, I don't recall doing this when I first switch to OpenWrt; but I had to use the Factory in Emergency Room, then flash again with sysupgrade after it booted. Otherwise it rebooted into the same unusable state - just FYI.)