I see this as being similar to the analogy of a snake as your lasting spiritual/true self. Each bend in the snakes body is a lifetime, and though it seems each bend comes to an end, actually they are all part of the continuing snake.
Yea! deem not of change: ye shall be as ye are, & not other.
You're not actually changing, cause the real you is bigger than this little curve. Sometimes you curve up, sometimes curve down, but always still the same old snake. Eternal and unchanged (interesting to think about this in connection with the 2 of Disks, the crowned (Kingly) snake of Change)
Therefore the kings of the earth shall be Kings for ever: the slaves shall serve.
The kings (lower case) of the earth (also lower case) shall be Kings (upper case) for ever. i.e. the king of the earth (the lower case lettering denoting a lesser importance to such a title) is a King (upper case denoting importance) for ever, as they achieve awareness of their own inherent Kingship during their earthy life. i.e. are not lost in the illusion of slavery.
The slaves (lowercase) shall serve (I notice the absence of "forever" compared with the King, and read this as a limited period of "slavery" as the true nature of us all is a King (upper case). So we can choose to identify with our bigger Kingly snake self, or just focus on the one little curve which may be a king or a slave.
There is none that shall be cast down or lifted up: all is ever as it was.
So doesn't matter if you're a king or a slave in your incarnation, as the real you is a King anyways and that doesn't change.
Yet there are masked ones my servants: it may be that yonder beggar is a King. A King may choose his garment as he will: there is no certain test: but a beggar cannot hide his poverty.
And because we are Kings, we can choose how we want to dress in our incarnated lifetime. A King may dress as a king or a begger/slave, which ever he feels like. If we were a Begger in reality, we would have no choice, so thankfully this is not the case.
I'm not sure if what I've typed in anyway makes sense or conveys what I meant to convey at least
Oh well, I tried!