So... if you want to use LUKS In-Place Conversion Tool, the notes below on converting a shipped-with-Ubuntu Dell XPS 13 Developer Edition (2015 Intel Broadwell model) may help you. There were a couple of small learnings to be had...
The page http://www.johannes-bauer.com/
For using the tool I booted from a 14.04.2 LTS USB live image and operated there, including downloading and compiling luksipc in the live session. The exact reason of resizing before luksipc was a bit unclear to me at first so I simply indeed resized the main rootfs partition and left unallocated space in the partition table.
Then finally I ran ./luksipc -d /dev/sda4 etc.
I realized I want /boot to be on an unencrypted partition to be able to load the kernel + initrd from grub before entering into LUKS unlocking. I couldn't resize the luks partition anymore since it was encrypted... So I resized what I think was the empty small DIAGS partition (maybe used for some system diagnostic or something, I don't know), or possibly the next one that is the actual recovery partition one can reinstall the pre-installed Ubuntu from. And naturally I had some problems because it seems vfatresize tool didn't do what I wanted it to do and gparted simply crashed when I tried to use it first to do the same. Anyway, when done with getting some extra free space somewhere, I used the remaining 350MB for /boot where I copied the rootfs's /boot contents to.
After adding the passphrase in luks I had everything encrypted etc and decryptable, but obviously I could only access it from a live session by manual cryptsetup luksOpen + mount /dev/mapper/myroot commands. I needed to configure GRUB, and I needed to do it with the grub-efi-amd64 which was a bit unfamiliar to me. There's also grub-efi-amd64-signed I have installed now but I'm not sure if it was required for the configuration. Secure boot is not enabled by default in BIOS so maybe it isn't needed.
I did GRUB installation – I think inside rootfs chroot where I also mounted /dev/sda6 as /boot (inside the rootfs chroot), ie mounted dev, sys with -o bind to under the chroot (from outside chroot) and mount -t proc proc proc too. I did a lot of trial and effort so I surely also tried from outside the chroot, in the live session, using some parameters to point to the mounted rootfs's directories...
I needed to definitely install cryptsetup etc inside the encrypted rootfs with apt, and I remember debugging for some time if they went to the initrd correctly after I executed mkinitramfs/update-initramfs inside the chroot.
At the end I had grub asking for the password correctly at bootup. Obviously I had edited the rootfs's /etc/fstab to include the new /boot partition, I changed / to be "UUID=/dev/mapper/myroot / ext4 errors=remount-ro 0 ", kept /boot/efi as coming from the /dev/sda1 and so on. I had also added "myroot /dev/sda4 none luks" to /etc/crypttab. I seem to also have GRUB_CMDLINE_LINUX="cryptdevice=/dev/sda4:myroot root=/dev/mapper/myroot" in /etc/default/grub.
The only thing I did save from the live session was the original partition table if I want to revert.
So the original was:
Found valid GPT with protective MBR; using GPT.
Disk /dev/sda: 500118192 sectors, 238.5 GiB
Logical sector size: 512 bytes
...
First usable sector is 34, last usable sector is 500118158
Partitions will be aligned on 2048-sector boundaries
Total free space is 6765 sectors (3.3 MiB)
Number Start (sector) End (sector) Size Code Name
1 2048 1026047 500.0 MiB EF00 EFI system partition
2 1026048 1107967 40.0 MiB FFFF Basic data partition
3 1107968 7399423 3.0 GiB 0700 Basic data partition
4 7399424 467013631 219.2 GiB 8300
5 467017728 500117503 15.8 GiB 8200
And I now have:
Number Start (sector) End (sector) Size Code Name
1 2048 1026047 500.0 MiB EF00 EFI system partition
2 1026048 1107967 40.0 MiB FFFF Basic data partition
3 1832960 7399423 2.7 GiB 0700 Basic data partition
4 7399424 467013631 219.2 GiB 8300
5 467017728 500117503 15.8 GiB 8200
6 1107968 1832959 354.0 MiB 8300
So it seems I did not edit DIAGS (and it was also originally just 40MB) but did something with the recovery partition while preserving its contents. It's a FAT partition so maybe I was able to somehow resize it after all.
The 16GB partition is the default swap partition. I did not encrypt it at least yet, I tend to not run into swap anyway ever in my normal use with the 8GB RAM.
If you go this route, good luck! :D