Im currently levelling a shadowblade and have messed up my spec in one line .
if i use my single line respec at 40 will my stealth autotraining stop ?

I wont add any points into stealth till level 48 !!

You can use single respec as full one without any fear of losing AT
The AT is succesfull in the moment you ding a level mutiple of 4, and your AT line is not on par with your level / 4.
This for exemple allows you to get your free points at level 40, respec and put stealth beyond AT value, go to wilton, pa and back stab enemies and then respec again at 43.9, taking stealth to its minimum possible value (in this case 10) to get the free spec points (11) as you ding level 44