you have some good advice here and a good plan, just want to add, that in the event someone did take on the challenge of opening up the machine to replace the original hard drive, it would be a better option to replace it with a faster SSD, as large as you can afford, and just split the Fusion drive and use the two internal SSDs individually, or as a JBOD RAID set. In any case, you would have to perform some Fission to split the Fusion drive when replacing the original. Also, they do make brackets so you can install a 2.5" SSD into a full size drive 3.5" bay, just an FYI.
Going to all the trouble to just increase storage space is not as good a reason as compared to if you want to increase the read/write performance/speed of the system. If you can boot and launch apps much faster on a new larger internal SSD, then you have a little bit more enticement to taking on that challenge. But for increased mass storage alone, speed is not such a big deal, and external drives are much simpler option to managing your data.
Keep a full backup of that internal Fusion drive, if one of the component drives fails, the whole system goes down. Have a bootable backup copy on an external drive, it will save your bacon later.