I THINK, overall, the threat is meant to be serious? For one thing, Gillenormand could almost certainly choose to disinherit Marius in favor of Mlle. Gillenormand, at least, without too much trouble. For another, the bulk of the actual inheritable estate seems to lie with Mlle. Gillenormand, right? And not Gillenormand himself. And when she's thinking of leaving the money other than with Marius, it's presented as a serious issue, but she's not using it as a threat, except in her own head.

So if it wasn't possible, I think Hugo THOUGHT it was possible? Probably it was like a lot of law-- not easy, but possible with the right hired experts. And Gillenormand's definitely in a position to unleash the bees and hounds--er, lawyers.