You can switch between same lvl of skills (same color) any time, it doesn't consume anything.

So you can switch from Sasuke's chase +1 to Y+2 and vice versa, you can switch from +1 mystery to Y and from +2 to Y+2 without any losses.

If you use skill vials and get something that doesn't have counterpart with matching color (so Y+1 mystery, Y and Y+1 chase), you won't be able to switch without losing access to that skill.