As you progress, older combat arts will "gray out" to you. When this happens, they are less effective against the newer higher level mobs you're facing.
Also these similar arts share the same reuse timer so the older versions are really of no use to you any longer.
Each time I get an upgrade art, I tend to replace the existing art with the new one using the same spot on the hotbar. After all, you're going to use it in the same situations so it might as well be in the same spot.
I also move them off of the first pages of my spell book, keping the first 2 or 3 pages for all the spells and arts I am currently using.
Don't worry about having spent mony on upgrading these arts to adept quality since these older, (lower level), arts will again become valuable when you start to mentor down to play with friends who have lower level characters or alts.
For example, a while back, a friend of mine wanted to level up a druid so I helped her out by mentoring down with a couple of my toons till she could pull her druid/fury up to a level where she could play with my mains.