It could be if they can forcibly feed off of love for another. Once the kingdom is conquered then you could harvest the ponies love for each other.
It doesn't matter though since it seems unlikely they would need to invade either way. Under either scenario they should be able to set down in a specific region and keep feeding off the locals forever. Migration would be an unnecessary and risky endeavor.
The invasion only makes sense if feeding off love is not sustainable. In that case the hive would be under pressure not only to find new sources of love but to do so quickly.
A direct attack may be necessary because infiltration of such a large group would need to be done slowly to avoid suspicion. With a group like Chrysalis's by the time everyone was set up you'd be facing starvation.
Perhaps small bands of changelings attack by infiltration and reproduce to form larger bands. In growing the group becomes more difficult to conceal and has more difficulty infiltrating new societies. This means that a successful band must go after larger societies(easier to bend into) and eventually resort to force.