Replace one disk, let the raid rebuild. Do the same with the other disk. Do an mdadm grow, then maybe fdisk / lvm / resize fs depending on your setup. Don't forget to install a bootloader when you put a new disk in.
Making a new array and migrating data is for chumps.
Oh, so easy?! If the RAID rebuilds the disk, why should I install a bootloader? Isn't it already in the rebuilt disk? By the way...hemmm...how do I install the bootloader? ๐ฃ
Yes it's really that easy. Raid in Linux is usually at the partition level, not the whole device. The bootloader resides in the first few blocks of the disk before your partitions, and isn't included in the raid.
Use grub-install on the new disk device, ie /dev/sda