Hi Xavier,
[quote user="Xavi García Maranges"]I'm trying to change the level number of a element[/quote]
This is wrong and it's not how MicroStation works.
What your code does is that is tries to change a number of level, that the element is assigned to. In other words, it tries to change level definition.
So the question is: Do you want to change a number of level or to move an element from one level to another?
I don't remember what MicroStation version do you use, but to work with level numbers is generally not recommended and I guess from V8 XM Edition it's treated as bad practice. In default MicroStation configuration it's not guaranteed the level number exists, the most of levels have no numbers today.
With regards,
Jan