Change point clouds IDs with respect to a relationship defined in an array

Question

I'm a new user of Python.

I'm trying to change ID to points in a point cloud. My point cloud has already a classification, so each point has got different IDs. I have an array that relates these IDs to new ones, something like:

ID_relation [[1, 50], [2, 41], [3, 21], [4, 78], [5, 4]]

I can read IDs from the point cloud in this way:

old_ID = myfile.intensity

(with laspy)(in my file IDs are saved like intensity value, but it's not a problem).

I don't know how to change ID values with the array relationship.