is the above an alternative method to go from Lineageos 18.1 to Lineagos 19.1 or should the original procedure from Its_Vixano
The OP guide seems to be written for people coming from something other than Lineage. The first part of it sets up the Lineage recovery and formats user data.
Since I was coming from Lineage and already had the recovery, I figured I could skip those steps. The OP guide says you can skip formatting data if you're coming from Lineage and I figured the recovery can't have changed too much.
I don't know how much testing was done for a Lineage 18 -> 19 upgrade. It worked for me but I barely had anything installed when I did it. Make backups in case of data loss and all that.
Something I don't understand in the OP guide is the suggestion to use the copy_partitions script. It only seems useful if your phone is somehow in an odd situation where miscellaneous A and B partitions don't match? It can't hurt to run it I guess.
can you give a more detailed explanation?
My 18->19 upgrade process matches the 19 "OTA" process. Refer to the OP guide for screenshots and details.
1. Boot to recovery
2. Sideload the lineage zip
3. Reboot to recovery (changes the active slot)
4. Sideload gapps
5. Sideload Magisk
The lineage zip is like a "full OTA" for the stock ROM. Flashing it overwrites system, boot and vendor, hence the need to flash gapps and magisk again. But the recovery only supports flashing to the current slot, hence the reboot in the middle.
I have TWRP 3.5.2_9.2 and Magisk on my phone, however, TWRP should not be used for flashing 19.1 because of FBE build (what's that?).
FBE has to do with encryption. I found this that might help explain it.
Booting TWRP 3.5.2_9.2 (fastboot boot twrp.img) shows only garbage names under /sdcard plus a bunch of other places (but not all of /data). So I guess TWRP doesn't support FBE encryption? I'm not sure why that matters for flashing system partitions, but if any of lineage, gapps or magisk expects to write to places covered by FBE then clearly that's not going to work properly under TWRP.
Since you have installed TWRP over the Lineage recovery, you'll have to get a Lineage recovery back on your phone before you can upgrade. Using the provided recovery should work I guess? If not, you could use TWRP to flash the most recent Lineage 18 zip, which will also replace TWRP with Lineage recovery. Part of the reason I attempted the 18->19 upgrade in the first place is because I couldn't get the provided recovery to boot on my phone.
I hope that helps.