So you're asking what would happen if you tossed an important TM away and you needed it for something? Well my opinion is, firstly, you'd have to be not thinking straight if you did something stupid like that, and secondly, you can just raise a pokemon that can learn that specific move! It's as simple as that!
Agree,but if you lost a TM like dig you can get it to open for the regis by leveling up Trapinch to level 41 or level up Ninacada to level 45.