You could do this probably by overwriting the object.
Use the -s flag in a rewrite command. For example:
gsutil rewrite -s [STORAGE_CLASS] gs://[PATH_TO_OBJECT]
[STORAGE_CLASS] is the new storage class for your object. For example, nearline.
[PATH_TO_OBJECT] is the name of the object whose class you want to change. For example, pets/dog.png.