You can add linux-lts to the pacstrap command, but it will install both linux and linux-lts. This is perfectly fine, since you can specify which kernel you want in syslinux.cfg. You may then remove linux if you choose to do so. However, rather than reinstalling the entire system, consider simply installing linux-lts and optionally removing linux, modifying /boot/syslinux/syslinux.cfg, adding lts to the kernel and initrd filenames, e.g. vmlinuz-linux-lts and initramfs-lts.img. This should give you a working system that uses the lts kernel without reinstalling from scratch. ~Kyle