Converting Container[Y] -> Container[Z] where Z < Y

[Roy_FlippingCoin]

package main

import "fmt"

type Z interface {
	A()
}

type Y interface {
	Z
	B()
}

type A struct{}

func (a A) A() {}
func (a A) B() {}

type Container[T Z] struct {
	Value T
}

func main() {
	instance := A{} // satisfies both Y and Z
	var container Container[Z] = Container[Z]{Value: instance}

	// this is not allowed because container is not an interface value
	// if _, ok := container.(Container[Y]); ok {
	// 	fmt.Println("it does")
	// }

	// this is also not possible
	// var c2 Container[Y] = container

	_ = container
	fmt.Println("no luck")
}

Go Playground - The Go Programming Language

Having a generic container which takes any type parameter which satisfies Z, and where interface Y is a specialisation of interface Z. Is there any way to convert a Container[Z] into Container[Y] having the Value of the instance of Container[Z] actually satisfies interface Y?

I know I can make a new instance of Container[Y] and copy the values from the Container[Z] instance into it, but this seems so costly…