You can't upgrade soldered RAM at all.

No. If both RAM chips are soldered—which “soldered dual-channel RAM” means—you cannot upgrade either chip. The whole point of soldered RAM is that it’s permanently attached to the motherboard.

You might be confusing “soldered” with “dual-channel.” Dual-channel just means the two sticks work together for better performance. Whether they’re soldered or socketed doesn’t change that. If one is soldered and one is in a slot (common on some laptops), then you can upgrade the slot-only stick, but you’ll almost certainly lose dual-channel if the capacities or specs don’t match. And mismatched capacities often drop to single-channel anyway.

So the honest answer: if both are soldered, you’re stuck. If one is socketed, you can upgrade that one—but expect to lose dual-channel benefits unless you replace both (which you can’t if the other is soldered). Not worth it for most people.

If future-proofing matters, buy a laptop with socketed RAM.