It is unclear
This is the focus of a heated debate in the Paizo's community (more recently, here), as there are no rules that explicitly say whether or not the spell can still affect a target that has changed after the spell was already cast. All we got is that decisions are made when the spell is cast.
You make all pertinent decisions about a spell (range, target, area, effect, version, and so forth) when the spell comes into effect.
Subjects, Effects, and Areas
If the spell affects creatures directly, the result travels with the subjects for the spell's duration. If the spell creates an effect, the effect lasts for the duration. The effect might move or remain still. Such an effect can be destroyed prior to when its duration ends. If the spell affects an area, then the spell stays with that area for its duration.
But we don't know if changing the conditions actually changes the spell. What if my target moved beyond the spell's range? What if my target is now heavier than my spell is able to lift? What if one of my hasted allies moves more than 30 feet away? The community seems divided on this, but a majority agrees that, unless the spell's effect state otherwise, the targets only have to be valid at the time of casting.
Some spells specifically call out conditions for it to remain in effect, like Unwilling Shield:
If you and the target of the spell move out of range of each other, the spell remains active, but damage is no longer shared until you are once again within range of each other.
Based on my experience with 3.5 and Pathfinder, I would rule that the Levitate spell fails temporarily. Similar to what happens when you move out of the area of a spell that targets an area. So, you couldn't move yourself up or down, but you won't fall either, you just gotta lose some weight.
This doesn't mean that the spell is automatically dismissed, simply that you are no longer a valid target and the spell cannot affect you anymore. Once you become a valid target again, and the spell still has some duration left, it should be able to affect you again.