just about every ability in the game can be morphed once it reaches rank 4. When you choose to morph a skill, you're given two options to pick from. Each option will give the skill a different improvement.

For example, veiled strike is a nightblade ability that stabs the target with a magical dagger. At rank 4, you can morph this into either surprise attack or concealed weapon. Both of these abilities still stab the target with a magical dagger, but surprise attack changes the ability to cost stamina instead of magicka and causes it to reduce the target's armor and deal bonus damage when used from stealth. Concealed weapon, on the other hand, keeps the ability cost as magicka and causes you to move faster while sneaking when the ability is on your bar.

Once you morph a skill, it can continue to be leveled up to rank 4 again, but it cannot be morphed again.

You can go to www.esohead.com and use their skill calculator to see all of the morphs for each ability in the game.

http://eso-skills.com/

Using this you can preview what skills morph to, and maybe even plan your build