I made a recent discovery which I hope everyone here can benefit from.
The quick story: Early on, I targeted Jaggi as my first armor set, as it has Attack Up (S). I registered this as Set#1. Then I made Loc Lac set for transporting quests, and registered that as Set#2. A few hours later, I made two Attack decorations, put on Set#1, and gemmed to Attack Up (M).
A day later, I ran another transport quest, and put on LocLac. When I switched back to Set#1, the decorations had removed!
Lesson learned: by using Equipment Sets, you can set up different skills with the same armor. Just make sure you have enough decorations if you have multiple armors using the same type of decorations.