Awesome, thanks for the feedback.
I used the GNOME Disks and reset the passphrase with no issues, worked pretty smoothly, no issues since.
My L15 is still hummin along just fine.

I am in the same boat. My Lib15v4 Secure Boot. I thought came out of the box preconfigured with dual drives that I spec'd when I placed my order. I would like to make sure that both drives are properly encrypted.

luks cryptsetup- trying to change encrypt pw
I see the warning, about using GNOME Disks to change the pw. I went to the link https://tracker.pureos.net/T541, and checked for updates to the string. Seems that mladen posted an update back in June 2020 that this issue was resolved. "Debian bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=928893 indicates that this has been fixed."
We are now headed in Jan 2021, can anyone confirm this to be true? I would prefer not to be the "ginny pig".
I tried the terminal route first and ran into some issues and did not work out as described above. My drives are set up differently, I have two drives in this machine and not listed as sda's.
Before I dove down that rabbit hole, I figured I would revisit using GNOME Disks, as it would be far easier, if its safe.
I don't have a virtual machine set up yet to test it out on.
Feedback please.
12-30-20 update- used the GNOME Disks and reset the passphrase with no issues.