Can you explain why you want to do this?

You do understand you are down grading the amount of storage your system currently has and the setup you now have is as fast as the Samsung for booting and running most things.

Yes, having a larger sized SSD can give you slightly better performance on the things the Fusion drive has not placed on the Fusion SSD drive.

But, the real issue here is these less used files what is slowing your system? Not likely. So increasing the SSD's size in most cases won't offer any improvement here (either in a Fusion or non-Fusion setup).

If your goal is to increase the HD size at the sametime as you're short of storage space then I could see replacing both drives that make up your Fusion set (SSD & HD) you have now.

You'll need to break the Fusion set so you can copy or clone your HD data over to the new HD, then put your new HD in where the current HD is and replace the current SSD drive for your newer one (if you want a deeper fusion drive).

We work with very large CAD files and have found having a larger SSD in the Fusion drive set does help. But, everyday web surfing, writing, drawing or image editing you won't see any improvement. I could see deep video editing might benefit with a deeper Fusion SSD. But, anyone doing that kind of work is more likely to have a RAID'ed HD or SSD drive set.