Installer crashes when trying to upgrade 32-bit system to 64-bit

[Gervase Markham]

I was surprised when Ubuntu let me try and upgrade a 32-bit 14.04 to a 64-bit 16.04. Is that supported? If not, it should be stopped with an error early on. If it is supported, then, well, I got this crash. Of course, it may not be related, and the 32/64 thing may be a coincidence...

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubiquity 2.21.63.3 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 4.8.0-36.36~16.04.1-generic 4.8.11
Uname: Linux 4.8.0-36-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
CasperVersion: 1.376.2
Date: Tue May 30 14:54:56 2017
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash ---
LiveMediaBuild: Ubuntu 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215.2)
ProcEnviron:
 LANGUAGE=en_GB.UTF-8
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)

[Phillip Susi (psusi)]

Your disk is currently partitioned to boot in bios mode, but you booted the installer in EFI mode. A warning message prompted you that this could be a problem and recommended performing a bios mode install instead, but you chose not to. You either need to perform the bios mode install, or partition the disk using GPT and set up an EFI system partition to install in EFI mode. This will be done for you if you choose the "use entire disk" guided install option.

[Gerv (gerv-launchpad)]

"A warning message prompted you that this could be a problem and recommended performing a bios mode install instead, but you chose not to."

That's not entirely right. The warning message said that there would be a possible problem booting _other_ operating systems installed on the disk afterwards. As I had no other operating systems other than the one I was upgrading, I chose to continue. If in fact there are other reasons for not continuing in this situation, the message needs updating to be more clear and explain what to do differently . So I am reopening this bug.