*shrug* check your stats with both an item that is rank appropriate (at or below your rank) and the item that's above your rank you're thinking of wearing. Unless the item you are replacing is seriously below your current rank, you're usually better with the item below your rank than the one above your rank. so if you're rank 15 and you're replacing a rank 9 item and the only thing you can get ahold of is a rank 18 item, your stats will go up if you put on the rank 18 item. However if you're replacing a rank 12 item it's probably better to keep that one for a level or two then switch.

The gear interface will show the difference so try it out. If you've got the inventory space it's always better to switch exactly when you get better stats. Oh, and by "try it out" I mean actually equip the stuff and see what your attack/defense stats changed to. The red/green difference on the item stats themselves don't actually mean much since they don't take optimal into consideration (I wish they did - heck if they did people wouldn't equip non-optimal gear as much since they'd see that switching would bring a sea of decreased stats).