The system will always remember the xp earned to get an ability from unlock to morph. You can never undo this...the character will "remember" their xp on that ability forever, even if you recast and ditch it and come back to it months later.

Also, the system remembers the leveling for BOTH paths of a morph. If I morph Uppercut into Dizzying Swing and level it to, say, rank three, then decide to re-morph over to wrecking blow, all that leveling I did on Dizzying Swing is saved. If I morph back to Dizzy Swing some day, it'll be right where I left off.

You're a Nightblade. Most of your class morphs go down a specific line depending on if you're stamina or magic. And usually a morph is entirely obvious which you should take, or the distinction is tiny and you can take either and be okay. But feel free to recast and level and experiment with multiple variations. Recasting morphs is pretty cheap, even when the special discount period ends.