1. Do it from a LiveUSB
2. Move the small partitions out of the way (to the front of the disk)
there's a small chance your bootloader won't work anymore if you move the /boot, just keep a liveUSB ready, it should be rescuable with a few magic mounts, chroot, and a grub-install (or whatever else you use)
that said though, make sure you don't have anything important on that disk in case shit fucks up, the usual