The plan was simple, which should have been the first warning. Four 4TB disks were full, I had four 8TB disks on the desk, and TrueNAS will happily grow a mirror if you replace each drive in turn and let it resilver. Pull one, insert the bigger one, resilver, repeat. The pool expands once the last small disk leaves. No downtime, no copying, no second machine.
It mostly went like that. The bit nobody tells you is how long a resilver takes when the pool is doing actual work at the same time. Each pass ran for the better part of a day, and I was foolish enough to start the second swap before properly checking which physical bay mapped to which gptid. The labels on the chassis were from a previous life and lied to me cheerfully.
So I stood there with a disk half out, watching the web UI, with glabel status open in another tab to confirm the serial number actually matched the device I was about to remove. It did not, the first time. The one I had a finger on was a healthy member of the pool, not the degraded one I'd just replaced. I slid it back, breathed, and checked the serial against the sticker before touching anything again.
Lesson, which I already knew and ignored: match by serial number, never by bay position. Write the serials down before you start. The pool is 8TB of headroom larger now and perfectly happy, and I am one swap older.