What
@greyhud90 said. That plan is tricky, right from the start. If you side with the people, you turn against the nobles. If you aren't part of the problem, there is a chance you are part of the solution and we can't have that! It might work, if the people are right about to behead the nobles and that plan might be seen as a last resort. But even then, many nobles would chose to slaughter their own population into submission before yielding. And if that happens you've got yourself a civil war, followed by invasions. You somehow have to tempt the nobles into turning themselves in. What MC said: The nobles with bad lands and debts (no perspective, no future) might give up what they have for some "real" power. But the really influential ones would just side with the friendly enemy next door.
I see only one way how this might work out and you won't like it. (I certainly don't.) The hostile, influential ones have to lose their ability to act in the right moment, at the same time and their is one realistic way to achieve that. Simultaneously murder them. Sadly all of them and their families. Even if you kill the family heads, their sons, wives, daughters, brothers, nephews, niece, uncles, aunts, brothers and sisters in law, ... , gardeners, ... , newly appointed 13 years old guard, ... or least favorite pet would just become the new head and stand against you, right the next moment. (Okay, the chain will be severed somewhere before gardener, but still....)
And you can't even honey some family members beforehand, to replace the old head after his dead and join your side, because their are too many people involves and someone WILL talk and that's that.