I would, if necessary, unplug all the individual fuse wires and relay holders to change the fusebox, but I wouldn't recommend that you even try to do so - you don't seem too confident in wiring. It would be a daunting task.

The fusebox is in 2 parts - the "box" with the 4 mounts surrounds the part that contains the fuses and relays. To seperate the 2 parts, gently spring the sides apart (and more brutally if "gently" doesn't work, but be careful not to crack it). If you need to you can change one "box" for the other you've got to get around the diffences in mounting, provided the 2 boxes are the same size - suck it and see.

Springing the "box" and removing it by sliding it off the loom will at least allow you to offer it up to the car to check that it mounts OK if the hole sizes are different.