class X<T> {
    _t: T|null = null
}

class Y<T> {
    _y: T|null = null
}

type ReplaceT<V extends unknown<any>, NewT> = unknown // How to do this

const x = new X<string>
const y = new Y<string>

// Assume that `x` is a type with 1 generic

type T1 = ReplaceT<typeof x, number> // X<number> expected
type T2 = ReplaceT<typeof x, boolean> // X<boolean> expected
type T3 = ReplaceT<typeof y, boolean> // Y<boolean> expected

Playground

Is this possible?