What happens is, when you upgrade from low-rank to high-rank armor is you get to keep the
stats and slots, the def itself will also be upgraded to high-rank armor's equivalent def and you can use armor spheres to upgrade the def all the way up just as the high-rank armor's equivalent can.
Some things to consider when you reach the point where you can get high-rank armors:
1. Some high-rank armor sets have abilities that low-rank armor sets does not have
2. When mixing armors to get specific abilities, you require mixing both high-rank and low-rank armor to get them.
For Example Hammer Set I use;
Abilities : HG Earplugs, Sharpness +1, Focus
Weapon: Devil's Crush (P) +1 Handicraft (1 Slot)
Helm: Helios Helm+ : +1 Handicraft, +2 Hearing (3 Slots : +4 FastCharge)
Chest: Helios Mail+ : +5 Handicraft, +3 Hearing (No Slots)
Gloves: Yamato Kote+ : +2 FastCharge (3 Slots : +4 FastCharge)
Pants: Helios Coil : +3 Handicraft, +5 Hearing
Boots: Helios Greaves : +5 Handicraft, +3 Hearing
Charm: Dragon Talisman +2 Hearing (2 Slots)
Skill Tree:
+15 Hearing
+15 Handicraft
+10 FastCharge
You can see that both my pants and boots are using the low rank version, the reason is if I use the high rank version, pants and boots will be
Pants: Helios Coil+ : +2 Handicraft, +1 Hearing
Boots: Helios Greaves+ : +4 Handicraft, +4 Hearing
+6 Handicraft, +5 Hearing
vs
Pants: Helios Coil : +3 Handicraft, +5 Hearing
Boots: Helios Greaves : +5 Handicraft, +3 Hearing
+8 Handicraft, +8 Hearing
so I'll be short of +2 Handicraft, +3 Hearing if I use high-rank equivalent gear