Flex mode works, but it's not perfect dual-channel.

Yes, you can replace the slot module with a larger one. The laptop will run the matching portion in dual-channel and the remainder in single-channel — that’s called flex mode, and most modern Intel/AMD laptops support it.

So if you have 8GB soldered and you put a 16GB stick in the slot, you’ll get 8GB+8GB in dual-channel (the first 8GB of each) and the remaining 8GB from the bigger stick in single-channel. That’s still better than 8GB soldered + 8GB slot in full dual-channel, because you end up with more total memory. But it’s not as fast as having two matching sticks.

If your workload is memory-bandwidth heavy (gaming, video editing, compiling), the loss from single-channel on the leftover portion might sting. If you’re just browsing or doing office work, you probably won’t notice.

You’re better off matching the soldered size exactly if you can. But if not, the bigger stick still wins.