If you have the space available, you can remove the drive you want to replace first.
But you could absolutely add the new drive, and then remove the smaller drive from the pool afterwards.
However, what I recommend here:
Remove the drive from the pool. Use the "Duplicate files later" option. This will remove the drive quickly, as it's leaving the duplicates on the drive. After the disk is removed, it will recheck the duplication status of the pool, and reduplicate files as needed.
Before it starts doing that, physically remove the "old" drive, and connect the larger drive.
This way, when it starts duplicating the data, it will use the new drive most likely. This will minimize the reshuffling of the files on the pool.
