If it's the orignal scracthes that are coming back through then you could go back to the original Detailers to get it warranty checked, as you said.
However, if they didn't fix it in the first place, there's nothing to say that they will actually fix it the second time round!
We would like to think that a much better result could be acheived by machine polishing the car to remove the scracthes and then applying a couple of coats of Carnauba wax... it won't last for six years (in fact, no where near!) but, it will offer a much better overall look at with regular maintanance will offer much better protection against the elements, and of course, the scratches will be gone rather than masked, so you wouldn't have to worry about them coming back through again
Hope this helps