The technique of altering a theme to give it a different character is often called:

The right answer is thematic transformation.  

Explanation:

This is a musical technique in which a theme, more specifically a leitmotif, is developed by changing the theme using permutation, augmentations, diminution and fragmentation. It is no connected to the original theme, it is independent of it, and it is usually used to show what the characters have in mind, or demonstrate an important change of the idea, announce something as well.