Okay. The line is just an entity. You're not supposed to do anything with it, except watch it. When you run your routine, and undo, the text should move back, but when you run the one I changed from command to subst and undo, the text should move back and the line should disappear. That's what you're supposed to see. The real question is why does the line disappear?
Also, in your command line, your first argument is pt1. It's my understanding that pt1 is a coordinate. The move command requires as its first argument an entity. coordinates and entities are independent of each other and you can't move coordinates. What should be there is txtentity. Does that make sense.
As far as it being just a challenge for you to learn lisp, GOOD. I'm relieved at that. So, in moving on, what say you we figure out how subst, entmod and entupd will work for you? Honestly, I see this as a good step. Then, how are you doing with understanding IF and COND statements? After that, how about we tackle the issue of what happens if you miss your entities or if the wrong entities are selected?