[ubuntu] Manual Upgrade to 16.04b2 from 14.04

I am trying to do an upgrade in place using the live cd of the 16.04 beta2. I use LUKS, but otherwise nothing special.
The live boot disk still can mount my encrypted /, so I know I haven't lost everything yet.

During installation I map partitions as:

/dev/mapper/ubuntu-vg-root EXT4 / 491220MB
/dev/mapper/ubuntu-vg-swap_1 swap 8577MB
/dev/sda1 ext2 /boot 254MB
/dev/sda5 crypto 499849MB (physical volume for encryption)
boot loader destination as /dev/sda

I pick to continue without any reformatting and enter my prior useame + password and computer name.
I have it pull updates and 3rd party options during install.

It seems to install fine, but complains that some packages could not be reinstalled afterwards. Seems like a reasonable outcome.

When I reboot, it drops me to busybox shell and the luks partitions are not visible. It never prompts for luks password.

I guess my question could be sumarized as how do I get the installer to pick up an existing Luks volume for /?