My lockpicking is 100 and I am thinking about setting it to legendary so I can return my few perks back to the pool. I don't want to do this, though, if resetting it back to 15 will make lockpicking harder.
Yes. The higher your lock picking the longer your picks will last while trying to open the lock. If you reset back to 15, your picks will break very often, thus making it necessary to constantly try to remember and find where the hot spot is for picking the lock.
This will mean that it may take you a lot more lock picks before finding the hotspot of a difficult lock.
Depending on how high your level is before you set the lockpicking skill to legendary depends on how much harder it is to level up the skill again. So for instance, if you're level is 80 or above. When you try to level up your lockpicking again, it would take longer than it normally would if you were at a lower level.