It does it automatically.
The “Expand” button is sort of a relic of the past, before the default “autoexpand” feature was set on pools.
Don’t use “detach” for this.[1] Either “offline” → “replace” (if you require a reboot and physical swapping of drives), or just skip to “replace” if you have the available slots in your case to house all the drives.
“Detach” will downgrade the mirror vdev to a single disk stripe. ↩︎