[RKDinOKC]

One of the reasons I always have a bootable carbon copy backup of my boot drive on an external drive. I can boot from it with whatever system I had before an upgrade. Don't have to spend any time restoring anything and then re-upgrading. External drives are pretty cheap, just make sure it is as big as your internal drive.

It would have been no big deal to boot from your external drive, decrypt you stuff, copy it to the disk with the new system on it, reboot to the new system and re-encrypt you stuff.

Get an external drive and install the old OS on it, then copy your files/old apps to it and do what you need. And move it to the drive with the new system. After you get everything moved updated and working on the new system, erase the external drive and use it as a Carbon Copy Bootable Clone backup of your new system.

Time Machine is great for recovering a few files, but pales in comparison the to usefulness of a bootable backup. I do both.
Time Machine also really needs more than 2 times the external space as the drive you are backing up to work properly.
Especially if you use something like Outlook for mail that makes huge mailstore files.