Honestly, it would really just come down to how many skill points you have laying around at the time. Early in your character's life you'll probably have to pick between healer and damage because you lack the skill points to spread around, but eventually you should have enough that you can put points into your heals and your damage stuff. Then, as long as you kept your healing weapon/lines/whatever leveled up appropriately, a quick gear change would make you a viable healer right off the bat.

Even easier for a magicka character. Already in light armor, already focused on magicka; just swap on the resto staff, swap a few skills around and tada! One healer, ready to go!