apply transformation only to some elements of a list but keed the list organisation

Hi !

Say i have a list (or tree) and i want to modify only some of the elements yet after i'm done with it, i want to have the exact same list/tree structure and indexes of elements (meaning the altered versions of the modified elements should take their place, and the elements that weren't modified should stay put in their positions).

One way is to apply the transformation to all elements, but to those that you want unmodified to give inputs that make the result the same as the initial stage (ex: 0 at the angle of rotation). I am looking 'though for a more elegant way of doing things (if there is one), so that no "ghost" transformations are needed.

Thanks for taking the time!