Or, y'know, they're banking on the Mane 6 to fight each other in the ensuing brawl as well.
Also, I doubt a regular drone could change into anyone "powerful." That's why only Chrysalis took on the form of Cadance; if it was unnecessary for her to be the one to do so, she had a near-infinite pool of minions who could have done so in her stead.
I'd say it's more likely that, with whatever amount of energy those changelings had, they were planning to use in-fighting and superior numbers to win that fight. Thinking about it, I can't recall a single changeling that looked like Rarity or Twilight ever using magic, so it's likely they couldn't.