See https://bugzilla.suse.com/show_bug.cgi?id=1094963#c35

Symptom:
Using the offline upgrade feature of the installation medium to upgrade Leap 42
with an encrypted data partition such as /home crashes YaST when selecting the
previous installation.

Reason:
Encrypted data partitions are listed in /etc/fstab by device mapper name, eg.
/dev/mapper/cr_home. In the installation environment YaST cannot relate that
path to autodetected volumes (boo#1094963).

Solution:
Prior to upgrading, change /etc/fstab to mount the encrypted partitions by UUID
instead of by path. For example if /etc/fstab contains

  /dev/mapper/cr_home /home xfs nofail 0 2

Use the following command to determine the UUID:

  # blkid|grep cr_home
  /dev/mapper/cr_home: UUID="12345678-1234-1234-1234-1234567890AB" TYPE="xfs"

Then change the in fstab line to

  UUID=12345678-1234-1234-1234-1234567890AB /home xfs nofail 0 2

After this modification of /etc/fstab yast can perform the upgrade without
crashing.