add_theme_support and remove_theme_support

I´m playing with a child theme, on parent i have defined many image_sizes, and i want to create a child functions to modify the parent sizes. Documentation says something about remove_theme_support, so i tryied removing the post-thumbnail feature on child and then adding it again in order to re-build sizes via new add_image_size values. But nothing, still gets the parent ones.

Any idea on something like this?