Question regarding Cubes and Potential Locks

Let's say I have an Epic Potential Weapon with +6% M.ATK on one line, and I locked it with a Potential Lock. If I were to Cube that item, and the tier level were to go up, would it still stay?

I believe it would stay. I don't think anyone has tested it enough to find out because if you're locking, Unique lines are generally the desired tier.

Given that M.ATK 6% is a possible line for both tiers I would say that it'd stay.

It stays. I've had it happen to me too. I cubed a hat that had 6% luk and crap and crap at epic, cubed with a pot lock, revealed unique, and it was still 6%