Software A: BSD-3 clause software, I am not its author.
Software B: forked from A, a few files have been changed by me. There are a few file additions, these files have a single author, me.
I know that I can license new files as GPLv3 as they are 100% made by me. But can I put a GPLv3 notice (without removing the original BSD-3 notice) in files modified by me?