Hello, sorry if this is dumb. I'm a noob when it comes to this stuff. My only "experience" is from when an IT friend of mine set up my Linksys WRT1200AC AC1200 with LEDE 17.01.4 back in 2017. (He did everything for me so I'm basically starting from square 1 here.) Obviously it would be a good idea to update for security reasons since I'm so out of date.
Is a direct upgrade from my version of LEDE to the newest Open WRT possible/safe? If not, what steps/version would you recommend instead?
I was in your shoes with a different device, an even older OpenWrt firmware and I got it... So I would suggest you to do exactly what I did, that is:
start @ https://openwrt.org/docs/guide-quick-start/start to learn things
get specific info for your device @ https://openwrt.org/toh/linksys/wrt1200ac
search the forum for tips and tricks concerning your current firmware and/or device
take your decision to stay or upgrade
Hope it helps to move forward...
You have a dual partition device. Best is to first get familiar with the procedure to switch the boot partition. This makes recovery on failure easier for you.
Note: A flashed firmware will always be installed into the alternate partition, not into the one you have currently booted from.
if you want to manually switch partition to the alternate partition before flashing the firmware image. You will then boot into the previous firmware, assuming that the other partition still holds a vendor image (flashing „factory“ image from that vendor image will then overwrite your current v17 partition with v21). It will not ask to keep settings. If things go south during flashing, you can revert to the older vendor firmware, but not to your v17.
if you want to overwrite the alternate partition by flashing the „sysupgrade“ image from your booted v17 partition (in your case, the now overwritten alternate partition might have contained an original vendor image). If you choose that path, you will be asked, if you want to keep settings, do NOT checkmark „keep settings“, the v17 settings will not work properly with v21. There will be no more original vendor firmware on the device after that, just 1x v17 and 1x v21. If things go south, you could revert to your untouched v17 firmware.
Take a backup of the settings. Then flash the new image without keeping the settings. Boot the new image and restore settings by hand using the backup files as a reference only.